Which Number would come in place of the underline mark in the series. 3, 9, 21, ___ , 93

First difference is 6 and second is 12. But if we take third as 18, then fourth difference will be 24. Then 93 can not be the last term.

So we have to take the difference as 6, 12, 24 and 48. 

So Answer is 21+24 = 45