I need help and this is the 3rd time I am asking you can have extra points for the othe ones

steps below

Step-by-step explanation:

a,b in 2nd quadrant: sine is positive and cosine is negative

sin a = 3/4      cos a = -√7/4

cos b = -12/13    sin b = 5/12

sin(a+b) = sin a cos b + cos a sin b

             = 3/4*(-12/13) + (-√7/4)*(5/12)

             = - 9/13 - (5√7/48)

             = -0.9679

cos(a-b) = cos a cos b + sin a sin b   (can you try yourself?)