Question in Homework Help Novelguide: Help with prototype functions in c programming?

The two prototypes, after the main function,and outputs to the main function. Each prototype has to do two calculations. My problem is, the compiler keeps failing and it says it is not possible to do two different calculations in one prototype. Can someone explain to me how to put both these calculations under one prototype? The two calculations I have to do is r=sqrt(x^2+y^2) and theta=atan(y/x) in one prototype.