If u=f(x,y), where x = e^s cos(t) and y = e^s sin(t), show that:

(d^2 u)/(dx^2) + (d^2 u)/(dy^2) = e^(-2s) ((d^2 u)/(ds^2)+(d^2 u)/(dt^2))

Sorry that's really hard to read...don't know how to use the math tags yet. Those are second partial derivatives. How do I do this problem? It has something to do with the chain rule since it's in that section of the book.