Rectangle with a width of 1.2m and a length of 2.4m is submerged in mercury. bottom of the rectange is 5m below the surface. find : Force?

Find what force? If the rectangle is oriented horizontally in the mercury and is rather thin, then the force on both the top and bottom surface of the rectangle is

F = 13.6 * 1000kg/m³ * 9.8m/s² * 5m * 1.2m * 2.4m = 1.9e6 N

But if the rectangle is "standing tall" in the fluid, then the pressure at the top of the rectangle is

p = 13600kg/m³ * 9.8m/s² * 2.6m = 3.465e5 Pa

and at the bottom is

p' = 13600kg/m³ * 9.8m/s² * 5m = 6.664e5 Pa

and so the total force on each vertical face is

F = ½(p + p')A = ½(3.465 + 6.664)e5Pa * 1.2m * 2.4m = 1.46e6 N

Hope this helps!