Force due to fluid pressure

A window on the side of a 5 foot deep swimming pool is in the shape of an inverted parabola located at the bottom of the pool 2 feet below the surface. Find the force F of water on this window.

This formula is used: where w is the density of the fluid, h is the strip depth and A is .

I'm not sure if its supposed to be or just if someone can answer this part than I should be able to do the rest. However my friend says the first is u substitution but the 2nd one is integration by parts. What exactly is integration by parts?