Question

what must be subtracted from 7 x -5y+z^2 to get 7^2+3y^2-2x^2

Answer 1

let that number be a
now according to question
7x-5y+z^2-a=7^2+3y-2x^2
a=7x-5y+z^2-7^2-3y+2x^2