Hi Sagar,

Please use this platform to ask questions related to information about various entrance examinations, colleges or universities, etc in the future. 

Now to answer your question,

In 6 days, A and B both complete a work.

In 1 day, they both do 1/6 work.

In 9 days A does a work.

In 1 day, A does 1/9  work.

Therefore, in one day B does (1/6-1/9) work, i.e. 1/18 work.

Thus, B will take 36 days to complete the same work.

BEST WISHES!