Ok this one is pretty straight forward :) This is what we know:
1. 2.7GHz is how many vibrations in 1 second. (2.7 x10^9 Hz)
2. Light always has a constant speed 2.9 x10^8 meters per second.
Therefore to find out the length of one of those vibrations you just divide 2.9 x10^8 by 2.7 x10^9
which looks like this in your calculator:
290000000 / 2700000000
= 0.107m
= 10cm