How do I show that the change of variable:

x = rcosθ , y = rsinθ

converts the Laplace equation uxx + uyy = 0 to

urr + 1/r ur + 1/r2 uθθ = 0

Here is my understanding:

uxx is the second partial derivative of u with respect to x, also written as d2u / d2x, but what is the equation with u in it that I have to differentiate?