What is the missing term?

(10 х - 4 х2) - (7 х + ?) = 3 х - 6 х2

(10 х - 4 х2) - (7 х + ?) = 3 х - 6 х2

10x - 7x = 3x which is given,

Now you have - 4x^2 - ? needs to equal - 6x^2

write it as an equation:

-4x^2 - ? = - 6x^2

To solve for? add 4x^2 to each side:

-? = - 2x^2

Multiply each side by - 1:

? = 2x^2