> I made X=1, y=2, and z=3.

You don't really want to do this sort of thing when simplifying expressions, and it appears you didn't end up making use of it.

The problem is that subbing in numbers gets you just a numeric answer, and it might lead you to decide that variables you assigned to be 1 were unimportant as factors, i.e. 3xy + 2x / 5 here would evaluate to be 8/5 so you might decide this could just be 3y+2 / 5.