Problem Statement

Given a sentence containing multiple words, find the frequency of a given word in that sentence.

Construct a method named 'find_frequency' which accepts two arguments 'sentence' and 'word', both of which are String objects.

Example: The method, given 'Ruby is The best language in the World' and 'the', should return 2 (comparison should be case-insensitive).

Hint: You can use the method Array#count to count the frequency of any element in the given array. eg:

[9,3,4,9,5].count(9)