I assume you have doubts for the following equation/statement
Fp = Force due to average hydrostatic pressure = average pressure × area = (1/2)ρgh × (b×h) = (1/2)×ρ×b×h2×g
area we have to consider for pressure force Fp is the normal area to pressure because force = pressure / area.
In the figure Fp is marked. we have to consider the area normal to Fp which is a rectangular area (b×h), b is given as the width of tank and h is the height of water column