Math Linear System Help

Question

Find values of a and b so that the linear system shown has a solution of (-9, 4).

Equation One : ax+by=-16

Equation Two : ax-by=-56

Answer 1

ax + by = -16

ax - by = -56

2ax = -72

ax = -23

If x = -9 then a = 23/9

Can you do the same for b?