Angle x is acute, angle y is obtuse, cos x = 15/17

and sin y = 3/5. Find sin (x + y).

After doing the math, I found sin (x+y) to be 77/85.

The textbook tells me that the answer is 13/85.

I know 13/85 would be the answer if we were looking for

sin (xMINUSy), right?