I really can't seem to solve this question and have tried mulitple ways of doing this, so here's hoping some one can help me or give me hints.

It cost 14 dollars to manufacture a backpack. If the backpack sells at x dollars each, the number sold, n, is given by n = (7/(x-4)) +5(100-x). What selling price would maximize profit?

I can't seem to get critical point and the question says to "simplify" implying that my answer should be an exact value.