my plant has brown spots on the leaves... please advise.

one of my plants has a bunch of brown spots on the leaves i have read that this is due to either a p or k deficiancy or both, i have some plant food in my shed that is
12-55-6 would this be a good feed to give the plant if not what would be a good mix?

you should flush the plants with neutral water atleast 4 times the amount of soil or container its in, then let it sit for 2 to 3 days then give it a normal feeding