Verified answer

y^2 = (2x + 1)^2 = 4x^2 + 4x + 1

sum(y^2) = sum(4x^2 + 4x + 1)

= sum(4x^2) + sum(4x) + sum(1)

= 4 sum(x^2) + 4 sum(x) + n

And you've been told what sum(x^2) and sum(x) are.