A swimming pool is 5 feet deep at one end and 10 feet deep at the other, and the bottom is an inclined plane. The length and width of the pool are 40 feet and 20 feet. If the pool is full of water, what is the fluid force on each of the vertical walls?

I understand how to find the force on the wall but... I got help from a tutor and he had me figure out the force on the 2 ends of the pool. Why not the two sides (the trapezoids)? Or was he wrong to leave those out?