### Instruction: If $x-y=15$ and $xy=4$, what is the value of $x^2+y^2$?### Response: I notice that the expression $x^2+y^2$ can be rewritten as $(x-y)^2+2xy$.
This might be helpful, because I already know the values of $x-y$ and $xy$.
So I substitute those values and get $(15)^2+2(4)$.