Angle a lies in the second quadrant and angle b lies in the third quadrant such ath cos a = -3/5 and tan b = 24/7

determine an exact value for

cos(a+b)

and

sin (a-b)

I got cos(a+b) = cosacosb - sinasinb

=(-3/5)(-7/25) - (4/5)(-24/25)

= 117/125

i am not sure if this is right, but i am also completely lost for sin (a-b)