Especially in incision 2, when they find Mb, they have used: Mb = FA*aAq*2.5m*1.25.

Please confirm what the symbols stand for.
In particular what is a
_{A} and M
_{b} ?
You have not provided symbols for the reactions at A and B?
Do you know how to calculate these reactions?
You should start by writing the equation for the bending moments for 16.2 and 16.3, using x for the horizontal axis and starting with x=0 at A. Can you do this?
It will have the form M = some function of x and the loads to the left of x.