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.