Therefore, sina=4/5 and cosB=5/ 13.
So sin(a-b)=sinacosB-cosasinB=4/5 times 5/ 13-3/5 times 12/ 13.
=- 16/65
Cos(a+B)=cosacosB-sinasinB=3/5 times 5/ 13-4/5 times 12/ 13.
=-33/65
If you don't understand, please ask. If it is helpful, please adopt it. Thank you!