Here is a little different way to tackle the same problem. Forget for a moment about constants. Many formulations are possible for a single problem. In this case, the second equation is in fact the answer for one of the unknowns. Many formulations are possible for a single problem.
Many formulations are possible for a single problem.
We may learn a few things. First, there is much freedom in naming variables and putting equations together. Do not be surprised if a friend of yours solved the same problem differently. Second, when translating a word problem into the language of mathematics, it's quite normal to get more than 1 variable or more than 1 equation. When more than 1 equation results, the equations are called simultaneous and we talk of a system of simultaneous equations.