This question has already been solved but I need help on understanding the solution. How did they find this formula for h? Thank you

2.4 An irrigation channel has the cross section shown in the diagram below. A dam is placed across it up to a height of d=1m as shown where L=3m and m=2m. With the water level at the crest on one side of the dam and no water on the other side calculate the hydrostatic force on the dam. Further calculate the moment about the base of the dam under the above conditions. the dam (Answer: 11445 N, 4087.5 Nm ) A=22+31A=2.5m.tydrostaticpressureP=bthecentroidalcentroidaldistancefronsurfaceh=d(L+2m)3(L+m)h=(3+22)3(3+2)h=715mP=1,0009.011.10P=4,578Nm2HydrostaticforceF=PAF=4,5782.5F=11,445N Explanation Hydrostatic pressure force is Explanation Please refer to solution in this step