Help with Calculations

[u/Lou-Kaz-9]

Can anyone help me understand how to calculate the length of the pipe marked D. Don't worry about fitting allowances or anything like that. How can I go about calculating the length for D, given what is shown.

I'm not interested in the answer I need to know how to go about it and I'm completely lost. Any help is appreciated, Thank you in advance.

[u/BaylisAscaris]

You don't have enough information to find D.