There appears to be something missing- any attempt at all by yourself to do this problem! If you were given a problem like this, surely you were given some idea as to how to at least begin the problem! Do you know how to calculate the water pressure at a given depth? Since the pressure depends only upon the depth, it will be constant along a horizontal line. Find the width of the plate at a given depth, x, and multiply by the pressure at that depth. That, times "dx", representing an infinitesmal thickness of the line, gives the force on the line. Integrate that.