The question is, at best, unclear: is it "if A is a 3x3 matrix such that Ax=b has at least one solution for...etc.", or is it "If A is a non-singular matrix then Ax=b has at least one solution for every...etc."??
As it is it makes no much sense and it's hard to guess what were you actually asked, but I'll give it a try: I'm guessing they wanted you to show them a way to calculate the inverse matrix of a given invertible matrix A (Invertible because they told us it is or because we had to deduce it. This I can't say), but if this is true then you didn't answered correctly the question: You had to ACTUALLY show the matrix inversion algorithm and, perhaps, show them how it is doen and why it works, not just merelity quoting it.