Use perfect square factoring patterns y^2 + 16y + 64 = 0

Arrange your given equation to resembles the form
a^2 +2ab+ b^2 because this equals (a+b)^2

So we get:
y^2+16y+8^2=0

Now compare
y^2+16y+8^2 to a^2 +2ab+ b^2

So we got
y^2+2•8 y+8^2=0 which equals (y+8)^2