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

Sentence: 'a small white dog looking out a car window'. Find frequency of the letter 'l'
3

Sentence: 'a young man riding a skateboard near a forest'. Find frequency of the letter 'd'
2

Sentence: 'blurry black and white image of a city street'. Find frequency of the letter 'r'

3