Think of 6 spaces. In the first place, you have 27 choices of what to put. (alphabet + space)
In the 2nd place, you have 27 choices.
3rd place - the same
All of them - the same,
Then you multiply the possibilities in each space to get the number of possibilities.
27 x 27 x 27 x 27 x 27 x 27 = ?