Need help with a problem. Ax=b

We just started class and our first HW problem over a chapter we haven't gotten to yet has me stumped.

The instructor made this one up I believe, and the following is from the chalkboard;

[1 2 0 1 3 | 4 ] is the reduced row echelon form if [A.b] for the eqn.

[0 0 1 2 4 | -3] Ax=b. Find solution for x.

[0 0 0 0 0 | 0 ]

From what I can gather by reading ahead in the book is for an Ax=b situation the solution is given by x=A^-1 * b... or x equals the inverse of A times b. From what I have read, inverses can only be calculated from matrices that are square or nxn dimensions. Therefore, I am stumped.

Any help is greatly appreciated, even if you just show me a general method, so I can solve it on my own.