i would solve it this way.
1. All we have now is 1 group of boys and one group of girls : so total arrangement possible is 2! = 2
2. Group of boys (3)can be arranged in 3! ways : so total arrangements are 3! = 6
3. group of girls can be arranged in 3! ways : so total arrangements are 3! = 6
so the total number of ways are 2*6*6 = 72
This type of solving is also applicable for questions that ask for arranging a couple who cannot sit seperated.
Consider giving me kudos if my explanantion was helpful.