The hydrostatic force is a force exerted by a fluid on a submerged body. This force comes from the weight of the fluid acting down on the submerged body, so it can be derived from the equation weight (in this case hydrostatic force)=massg and mass=densityvolume. You can calculate the volume of the column of water above the object by multiplying the surface area of the object by the depth the object is submerged by. Calculate the mass of the object and density of the fluid and then substitute into the equation above to find the total force exerted on the submerged body.