Not sure if this is the right forum for this. I'm new here

I'm studying to take the GMAT and I'm stumped on the following problem:

5^21 * 4^11 = 2 * 10^n

I need to solve for n. The answer I believe is 21, but I don't know how to arrive at the answer without a calculator. So I'm asking how to solve this without actually calculating the 5^21 and 4^11.

All the review material I've been reading talks about having like bases before combining exponents and I'm not seeing how to get like bases in this problem.