Given sina= 3/4 and cosB = (-5)/13 and this is Quad II find cos(a+B)

Use the formula

cos (A + B) = cos A cos B - sin A sin B

It will help to know that
sin B = 12/13 (if B is in the 2nd quadrant), and
cos A = -(sqrt7)/4 (if A is in the second quadrant)
Those values can be obtained with
sin^2 + cos^2 = 1

You don't say which angle in in the second quadrant. Is it A, B or A+B?