Partial Differentiation 2

Sorry, i'm stuck, here goes....

In the first two parts of the question, you are asked to show that:

and

which i have done, and it's all fine and dandy :)

However, the last part confuzes me.... :(

If , show that:

The problem is i'm not sure what actually means. I understand it to be: f is a function of two variables, x and y, and f is a function of two variables, u and v, and these two functions are equal. But u and v are themselves functions of x and y so what is f?

A push in the right direction would be greatly appreciated! :)