Solve 3x^3-3x^2-18x

$$3 { x }^{ 3 } -3 { x }^{ 2 } -18x$$

Factor

$$3x\left(x-3\right)\left(x+2\right)$$

Solution Steps

Factor out 3.

$$3\left(x^{3}-x^{2}-6x\right)$$

Consider x^{3}-x^{2}-6x. Factor out x.

$$x\left(x^{2}-x-6\right)$$

Consider x^{2}-x-6. Factor the expression by grouping. First, the expression needs to be rewritten as x^{2}+ax+bx-6. To find a and b, set up a system to be solved.

$$a+b=-1 ab=1\left(-6\right)=-6$$

Since ab is negative, a and b have the opposite signs. Since a+b is negative, the negative number has greater absolute value than the positive. List all such integer pairs that give product -6.

$$1,-6 2,-3$$

Calculate the sum for each pair.

$$1-6=-5 2-3=-1$$

The solution is the pair that gives sum -1.

$$a=-3 b=2$$

Rewrite x^{2}-x-6 as \left(x^{2}-3x\right)+\left(2x-6\right).

$$\left(x^{2}-3x\right)+\left(2x-6\right)$$

Factor out x in the first and 2 in the second group.

$$x\left(x-3\right)+2\left(x-3\right)$$

Factor out common term x-3 by using distributive property.

$$\left(x-3\right)\left(x+2\right)$$

Rewrite the complete factored expression.

$$3x\left(x-3\right)\left(x+2\right)$$

Evaluate

$$3x\left(x-3\right)\left(x+2\right)$$