add the two equations together:

move things around:

factor:

factor again:

hmm, 7 is a prime number though and so cannot have two integer factors,unlessandorand (which means that in this case either a or c must be zero, I don't know if you consider that a positive integer)

In the first case: and

So if we add those two together:

In the second case: and

So if we add those two together:

So the answer is: