this question has been solved (https://chegg-study.app.link/f0ap5a2jfyb) but I'm not sure how the value of H_p was found. where does the 12 come from? Can you explain what formula this is? Thank you

A reservoir contains water to the depth of 3.0m. In one of the vertical sides of the reservoir there is a rectangular gate 1m wide and 1.5m deep, the lower horizontal edge of the gate being situated 0.5m above the bottom of the reservoir. The gate is pivoted about its upper edge. Calculate the moment necessary to keep the gate in a closed position. (Answer: 22103Nm ) FHFHhP=Hydrestaticforce=ghA{h=1+21.5==10009.811.75.1.5=25751.25N=h+AhIG=1.75+1.51.75(11.53)/12=1.8571m FBD of gate moment about pivot, 0 : M0M0=FH(hP1)=25751.25(1.85711)=220=22103Nm