Solve -x^2+4x+21=0

$$- x ^ { 2 } + 4 x + 21 = 0$$

Solve for x

$$x=-3$$

$$x=7$$

Steps Using Factoring By Grouping

To solve the equation, factor the left hand side by grouping. First, left hand side needs to be rewritten as -x^{2}+ax+bx+21. To find a and b, set up a system to be solved.

$$a+b=4 ab=-21=-21$$

Since ab is negative, a and b have the opposite signs. Since a+b is positive, the positive number has greater absolute value than the negative. List all such integer pairs that give product -21.

$$-1,21 -3,7$$

Calculate the sum for each pair.

$$-1+21=20 -3+7=4$$

The solution is the pair that gives sum 4.

$$a=7 b=-3$$

Rewrite -x^{2}+4x+21 as \left(-x^{2}+7x\right)+\left(-3x+21\right).

$$\left(-x^{2}+7x\right)+\left(-3x+21\right)$$

Factor out -x in the first and -3 in the second group.

$$-x\left(x-7\right)-3\left(x-7\right)$$

Factor out common term x-7 by using distributive property.

$$\left(x-7\right)\left(-x-3\right)$$

To find equation solutions, solve x-7=0 and -x-3=0.

$$x=7 x=-3$$

Steps Using the Quadratic Formula

All equations of the form ax^{2}+bx+c=0 can be solved using the quadratic formula: \frac{-b±\sqrt{b^{2}-4ac}}{2a}. The quadratic formula gives two solutions, one when ± is addition and one when it is subtraction.

$$-x^{2}+4x+21=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute -1 for a, 4 for b, and 21 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-4±\sqrt{4^{2}-4\left(-1\right)\times 21}}{2\left(-1\right)}$$

Square 4.

$$x=\frac{-4±\sqrt{16-4\left(-1\right)\times 21}}{2\left(-1\right)}$$

Multiply -4 times -1.

$$x=\frac{-4±\sqrt{16+4\times 21}}{2\left(-1\right)}$$

Multiply 4 times 21.

$$x=\frac{-4±\sqrt{16+84}}{2\left(-1\right)}$$

Add 16 to 84.

$$x=\frac{-4±\sqrt{100}}{2\left(-1\right)}$$

Take the square root of 100.

$$x=\frac{-4±10}{2\left(-1\right)}$$

Multiply 2 times -1.

$$x=\frac{-4±10}{-2}$$

Now solve the equation x=\frac{-4±10}{-2} when ± is plus. Add -4 to 10.

$$x=\frac{6}{-2}$$

Divide 6 by -2.

$$x=-3$$

Now solve the equation x=\frac{-4±10}{-2} when ± is minus. Subtract 10 from -4.

$$x=-\frac{14}{-2}$$

Divide -14 by -2.

$$x=7$$

The equation is now solved.

$$x=-3 x=7$$

Steps for Completing the Square

Quadratic equations such as this one can be solved by completing the square. In order to complete the square, the equation must first be in the form x^{2}+bx=c.

$$-x^{2}+4x+21=0$$

Subtract 21 from both sides of the equation.

$$-x^{2}+4x+21-21=-21$$

Subtracting 21 from itself leaves 0.

$$-x^{2}+4x=-21$$

Divide both sides by -1.

$$\frac{-x^{2}+4x}{-1}=-\frac{21}{-1}$$

Dividing by -1 undoes the multiplication by -1.

$$x^{2}+\frac{4}{-1}x=-\frac{21}{-1}$$

Divide 4 by -1.

$$x^{2}-4x=-\frac{21}{-1}$$

Divide -21 by -1.

$$x^{2}-4x=21$$

Divide -4, the coefficient of the x term, by 2 to get -2. Then add the square of -2 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}-4x+\left(-2\right)^{2}=21+\left(-2\right)^{2}$$

Square -2.

$$x^{2}-4x+4=21+4$$

Add 21 to 4.

$$x^{2}-4x+4=25$$

Factor x^{2}-4x+4. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x-2\right)^{2}=25$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-2\right)^{2}}=\sqrt{25}$$

Simplify.

$$x-2=5 x-2=-5$$

Add 2 to both sides of the equation.

$$x=7 x=-3$$

Steps Using Direct Factoring Method

Quadratic equations such as this one can be solved by a new direct factoring method that does not require guess work. To use the direct factoring method, the equation must be in the form x^2+Bx+C=0.

$$x ^ 2 -4x -21 = 0$$

Let r and s be the factors for the quadratic equation such that x^2+Bx+C=(x−r)(x−s) where sum of factors (r+s)=−B and the product of factors rs = C

$$r + s = 4 rs = -21$$

Two numbers r and s sum up to 4 exactly when the average of the two numbers is \frac{1}{2}*4 = 2. You can also see that the midpoint of r and s corresponds to the axis of symmetry of the parabola represented by the quadratic equation y=x^2+Bx+C. The values of r and s are equidistant from the center by an unknown quantity u. Express r and s with respect to variable u. <div style='padding: 8px'><img src='https://opalmath.azureedge.net/customsolver/quadraticgraph.png' style='width: 100%;max-width: 700px' /></div>

$$r = 2 - u s = 2 + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -21

$$(2 - u) (2 + u) = -21$$

Simplify by expanding (a -b) (a + b) = a^2 – b^2

$$4 - u^2 = -21$$

Simplify the expression by subtracting 4 on both sides

$$-u^2 = -21-4 = -25$$

Simplify the expression by multiplying -1 on both sides and take the square root to obtain the value of unknown variable u

$$u^2 = 25 u = \pm\sqrt{25} = \pm 5$$

The factors r and s are the solutions to the quadratic equation. Substitute the value of u to compute the r and s.

$$r =2 - 5 = -3 s = 2 + 5 = 7$$