pipes

[KHANYISO]

Hi all I, would lik eto be assisted on how to calculate pipe sizes on sewer network. The only information i have been given is the slopes.
I have to calculate Q= PWWF
Assume the diammeter , using Q= A/N * (A/P)....... formula
Compare the two Q'Z
read what graph

Tell m e if im on the right track, urgently please.

Thanks all for your response.