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9x^{2}-42x+49-5\left(2x-4\right)\left(x-2\right)=-\left(x+2\right)^{2}+4x+9
Use binomial theorem \left(a-b\right)^{2}=a^{2}-2ab+b^{2} to expand \left(3x-7\right)^{2}.
9x^{2}-42x+49-5\left(2x-4\right)\left(x-2\right)=-\left(x^{2}+4x+4\right)+4x+9
Use binomial theorem \left(a+b\right)^{2}=a^{2}+2ab+b^{2} to expand \left(x+2\right)^{2}.
9x^{2}-42x+49-5\left(2x-4\right)\left(x-2\right)=-x^{2}-4x-4+4x+9
To find the opposite of x^{2}+4x+4, find the opposite of each term.
9x^{2}-42x+49-5\left(2x-4\right)\left(x-2\right)=-x^{2}-4+9
Combine -4x and 4x to get 0.
9x^{2}-42x+49-5\left(2x-4\right)\left(x-2\right)=-x^{2}+5
Add -4 and 9 to get 5.
9x^{2}-42x+49-5\left(2x-4\right)\left(x-2\right)+x^{2}=5
Add x^{2} to both sides.
9x^{2}-42x+49+\left(-10x+20\right)\left(x-2\right)+x^{2}=5
Use the distributive property to multiply -5 by 2x-4.
9x^{2}-42x+49-10x^{2}+40x-40+x^{2}=5
Use the distributive property to multiply -10x+20 by x-2 and combine like terms.
-x^{2}-42x+49+40x-40+x^{2}=5
Combine 9x^{2} and -10x^{2} to get -x^{2}.
-x^{2}-2x+49-40+x^{2}=5
Combine -42x and 40x to get -2x.
-x^{2}-2x+9+x^{2}=5
Subtract 40 from 49 to get 9.
-2x+9=5
Combine -x^{2} and x^{2} to get 0.
-2x=5-9
Subtract 9 from both sides.
-2x=-4
Subtract 9 from 5 to get -4.
x=\frac{-4}{-2}
Divide both sides by -2.
x=2
Divide -4 by -2 to get 2.