write the equation that describes the 3,6,12,21?

Johlie,

Welcome to this great site, first!
It must have been 3, 6, 12, 24, I presume. Please check if it is really so as you claim or write here. If it is - 3, 6, 12, 24 it is simply doubling of the previous number. Is not it?

Hint: If you consider the difference between each of the terms in 3,6,12,21 you should see a simple pattern.

Yeah, ebaines has hit the nail on the head!

3,6,12 and 21...
6-3 = 3, 12-6 = 6 and 21-12 = 9
We find that the differences are in AP i.e. Arithmetic Progression. So the next difference should be 12, (increase by 3, as before), and so the next number/item/member will be 21+12=33.