How to express a standard deviation in terms of other variables' mean and s.d.?

This shouldn't be a too difficult problem, yet I don't know how to tackle it. Any ideas?

The average and the standard deviation of two independent probability variables x and y is (Ux, Uy) and (Sx, Sy), respectively. Assuming that the standard deviation of these two functions is infinitesimally small as compared to their respective averages, express the approximation equation Sz (where the probability variable Z = X/Y) in terms of Ux, Uy, Sx and Sy

Thanks!