Fill in the missing parts:

Question

(5x + ___)^2 = ___ · x^2+70xy + ___

Answer 1

recall :
(a+b)^2 = a^2 + 2ab + b^2

so for (5x + ...)^2 = ...x^2 + 70xy + .....
clearly the first term must be 25x^2
and, since the middle term contains a y, there has to be a y term at the end of the binomial
let the binomial be (5x + by)
the middle term would be 2(5x)(by)

then 2(5x)(by) = 70xy
10b = 70
b = 7

so you would have (5x + 7y)^2 = 25x^2 + 70xy + 49y^2