Reverse engineering a function

I'm not sure if this is the correct forum for my question, so please move it as needed.

It has been many years since I did any advanced math, and I'm having some trouble determining how to come up with a function for a given set of inputs. I have a compiled program but no source code, and I want to use the function in a new program. I can enter values into the existing program and get a result, but I don't know the function for calculating the output. For example, 40x + 60y + 200z = 42 and 30x + 60y + 300z = 58.

Should I create equations for 3 sets of coefficients with solutions and solve the set of simultaneous equations?