The complement of an event is basically the complete opposite.
If you define event A to be the event of selecting an R, then the complement of event A is not selecting R.
The probability of selecting R is 1/6 because there is 1 letter R out of 6 letters total. The complement of this is 5/6. You could count out 5 letters that aren't R, out of 6 total, or you can subtract 1/6 from 1 to get
Subtract 13.09 from 21.59 giving you 8.5. Since the amount of space on both the left and right margin have to be equal you would divide by 2 to get 4.25 on the left margin as well as the right.
