a(7x-x^2)-b(5x-4x^2)=-11x^2+31x,,a&b=?

a(7x-x^2)-b(5x-4x^2)
=7ax-ax^2-5bx+4bx^2
=(4b-a)x^2+(7a-5b)x

Since that equation is equal to -11x^2+31x,
So
-11=4b-a--(1)
31=7a+5b--(2)

7*(1)+(2):
-77+31=28b-7a+7a-5b
-46=23b
b=-2,
a=3//