The word SUMMER VACATION has 14 letters in total. So in that case, 14 would be your denominator. For the probability if you chose the letter ‘M’, you would need to count all the ‘M’(s) in SUMMER VACATION.
There are two ‘M’s in SUMMER VACATION so the total number of ‘M’s would be your numerator.
2/14 is your answer. 2 (letters of M) / 14 (all letters in total)
If you want to simplify it further then the answer would be 1/7.