Question 571319
x^2-2x+5x-(-x^2+7x)


x^2-2x+5x+x^2-7x


(x^2+x^2)+(-2x+5x-7x)


2x^2-4x



So x^2-2x+5x-(-x^2+7x) = 2x^2-4x for all values of x