GMAT Prep ques.

This one is probably easy for you guys, but it is stumping me.

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

Reads: 5 to the 21st power mult. by 4 to the 11th power equals 2 mult by 10 to the nth power. What is n?

I know the answer because I took the practice test, but how does one get there?