I do not know what calculation/formula to use to show that something is an independent risk factor.

E.g., in a cohort study:

Group A (n=1000) - exposure to X - outcome exposed 70/1000 demonstrate Z

Group B (n = 1000) - no exposure -- outcome control 23/1000 demonstrate Z

However, of Group A 750 also exposed to Y -- 60 demonstrate Z

Group B 300 also exposed to Y -- 16 demonstrate Z

How do I show that Y is an 'independent risk factor' for Z.

(I know that I can calculate the Risk Ratio to compare Exposure X to control, but do not know what formula I use to show that Y is an independent risk factor for Z).