This isn't a homework problem but I found it on the internet and can't figure out how to do it. It's one of those "divisible by whatever"-type problems which I never learned how to solve... I don't know how to work with this divisibility stuff when it's generalized to numbers like x+y+z. I don't even know where to begin. All I can do is "9w = x+y+z for some number w"...and now I don't know where to go. If I think up example values for x,y, and z I find that (a) keeps working, but I don't know why. Any help appreciated!