How do I work out stress/strain using mass instead of force?

Stupid question, and the answer is probably right in front of me, but I am absolutely stumped at this and can't seem to find anything on it. Any kind of help would be appreciated
(preferably in the context of A-level physics )

FYI this is the question I'm stumped on, if I get this then I can do the rest;
Mild steel has a breaking strength of 500MPa. If you want to support a 200kg piano using a single steel wire, what is the minimum diameter of wire you require? Give your answer to 2 significant figures.