If you add 13 to each side, that wouldn't be correct because that is not how you get rid of the 13 that is being added to m. If you want m by itself, ask yourself : what is the opposite of adding 13 to x? It would be subtracting 13, to get 0, leaving m by itself. So if you subtracted 13 from that side, you have to subtract it to the other side, so then 56 subtracted by 13 would be 43. In conclusion,
x = 13. Hope this helps! Let me know if you still don't get it. Glad you're feeling better! :)