Hi. I'm going through my old trig book, for review. I am stumped by this problem, not being sure what approach to take.

Of course, cos(a+b) = cos a cos b - (3/5)(5/13), but I'm not sure where to go from there. I guess I could use a calculator to get the arcsines, but I don't think that is how I am supposed to solve the problem.Find the exact value of cos(a +b) if sin a = 3/5 and sin b = 5/13, with a in quadrant II and b in quadrant IV.