The vertical projection is a rectangle with height 1.8 m and width 1.2 m. The depth of the centroid of this rectangle is $5.5 - 1.8/2 = 5.5 - 0.9 = 4.6$ m.
The horizontal force is given by:
$$F_H = \rho g h_c A$$
where $\rho = 1000 kg/m^3$ is the density of water,
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