The two complex numbers 5+a + 4i and 3 +2bi -3i are equal. For two complex numbers to be equal both their complex as well as real components have to be equal.
This gives: 5+a = 3 and 4 = 2b - 3
=> a = -2 and b = 7/2
The required value of a = -2 and b = 7/2.