Definition: In this task, you need to reverse all words of a given length in the sentence. The number of letters in a word determine its length. For example, the length of the word "apple" is 5.
Input: Sentence: 'a man is sitting on a couch on a laptop while a woman works behind him on  a desktop'. Reverse all words of length '6' in the given sentence.

a man is sitting on a couch on a potpal while a woman works dniheb him on a desktop