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

[Q]: Sentence: 'a black bear sitting in the grass taking a rest'. Find frequency of the letter 'a'
[A]: 6


[Q]: Sentence: 'a black computer with a screen reading "press any key to begin."'. Find frequency of the letter 'a'
[A]: 5


[Q]: Sentence: 'a man on a court swinging a tennis racket'. Find frequency of the letter 'n'
[A]:
6