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 street scene looking at the back of the stop sign'. Find frequency of the letter 't'
Example Output: 6

Example Input: Sentence: 'a young man holding a yellow frisbee on top of a lush green field'. Find frequency of the letter 'o'
Example Output:
6