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

Example Input: Sentence: 'a large motorcycle that has a man dressed in old clothing next to it'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a toilet sits in a cluttered bathroom next to a sink'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'two men who are petting an elephant that is in an enclosure'. Find frequency of the letter 't'
Example Output:
6