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