Given that A and B are acute angles with cos A = 4/5 and sin B = 5/13, find without using tables or a calculator

cos (A + B)

Here is my take on the question.

cos (A + B)

= cosAcosB - sinAsinB

= (4/5)cosB - sinA(5/13)

After this, I don't know how to continue. Answer is 33/65