Rewrite without parentheses and simplify.
(y+7)^2

(y + 7)^2 = (y + 7)(y + 7)

Multiply all the terms in the first parenthesis to all the terms in the 2nd parenthesis:

y * y = y^2
y * 7 = 7y
7 * y = 7y
7 * 7 = 49

So we have:

y^2 + 7y + 7y + 49

Combine like terms:

y^2 + 14y + 49