Since it is divisible by 72, it should be divisible by 9 and 8.
In order for the number to be divisible by 9, it should have digits with a sum which is a multiple of 9. So,
A + 4 + 2 + 7 + 3 + B = A + B + 16, this is divisible by 9.
For the number to be divisible by 8, the last 3 digits must be divisible by 8.
73B is divisible by 8 therefore 73B is 736.
B = 6
A + B + 16 = A + 6 + 16 = A + 22 is divisible by 9 therefore A + 22 = 27 so A = 5.
Therefore A = 5 and B = 6.
