Hint:
I would begin by factoring as follows $$(m-8)(n+3)=59-24=35 = 1\cdot 5\cdot7$$
Now can you think of the possibilities for $m-8, n+3$? Do not forget $m, n > 0$
* * *
_P.S. Modular arithmetic seems really unnecessary here, though it wouldn't be hard to contrive its use to solve._