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

Example Input: Sentence: 'a young boy bouncing a tennis ball on a tennis racquet'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a plate of food, with broccoli and nuts'. Find frequency of the letter 'b'
Example Output: 1

Example Input: Sentence: 'man in a radio studio wearing a suit in front of a microphone'. Find frequency of the letter 't'
Example Output:
3