In this task, you need to count the number of times the given letter appears in the given sentence.

[EX Q]: Sentence: 'a man riding a white surfboard on a large wave'. Find frequency of the letter 'a'
[EX A]: 7

[EX Q]: Sentence: 'a photo of a couple of people at a convention'. Find frequency of the letter 'v'
[EX A]: 1

[EX Q]: Sentence: 'a young boy with a tennis racket walks by the net'. Find frequency of the letter 'e'
[EX A]:
4