let the 7' base be the x-axis and let the y-axis bisect the vertical cross section.
the right side of the cross-section has equation
sketch in a horizontal "slab" of water in the tank ...
its width is
its length is 16
hence, the area of a representative slab is
the thickness of a representative "slab" is
work lifting a representative "slab" = (weight-density)(area of slab)(distance the slab is lifted)(slab thickness)
the work required to lift a single "slab" is ...
all the horizontal "slabs" of liquid reside from y = 0 to y = 8 forming the limits of integration.