If $x + 2y= 4$ and $xy = -8$, what is the value of $x^2 + 4y^2$?
I notice that the expression I want to find, $x^2 + 4y^2$, can be rewritten as $(x + 2y)^2 - 4xy$. This might be helpful, because I already know the values of $x + 2y$ and $xy$. I get $(x + 2y)^2 - 4xy = (4)^2 - 4(-8) = 16 + 32 = 48$. This is the value of $x^2 + 4y^2$. 