Namely: sin(a-b)cos a- cos(a-b)sin a=3/5.

sin(a-b-a)=3/5

De: sinb=-3/5

Cosb=-4/5 because b is the third quadrant.

Therefore, sin (b+5π/4) = sinbcos (5π/4)+cosbsin (5π/4).

=(-3/5)(-√2/2)+(-4/5)(-√2/2)

=7√2/ 10

Have fun! I hope I can help you. If you don't understand, please ask. I wish you progress in your study! O(∩_∩)O