I have a feeling that this question has been answered many times before, but here goes:
10 = 2*5 and this is your starting point.
You have that 4^11 over there that might confuse you. You get that 4^11 = (2^2)^11 = 2^(2*11) = 2^22. Now, you must multiply this with 5^21 and you get that 5^21 * 2^22 = 2 * (5^21 * 2^21) = 2 * (5*2)^21 = 2*10^21.
This will make n = 21.

DanaJ

Can you please explain how

5^21 * 2^22 = 2 * (5^21 * 2^21)

thanks

kaf: Sure.
Just look at 2^22: it's 2^(21+1) or 2^21 * 2^1 = 2^21 * 2.
This means that 5^21 * 2^22 = 5^21 * 2^21 * 2 and then you just put the two in a parenthesis, giving you 2*(5^21 + 2^21).