I can't figure this out and I feel stupid. Help me out here:

Multiple choice, no calculator allowed:

If [2^x]-[2^(x-2)]=3(2^13), what is the value of x?

a) 9

b) 11

c) 13

d) 15

e) 17

The answer is d, but how do you solve this without a calculator?

edit: Nevermind, got it.