Representation of data — practice question

The diagram presents the cross-section $OABCDE$ passing through the centre of mass of a uniform prism resting on a rough inclined plane. The section $ADEO$ forms a rectangle with $AD = OE = 0.6\,\text{m}$ and $DE = AO = 0.8\,\text{m}$. The section $BCD$ is an isosceles triangle in which angle $BCD$ is a right angle, and $A$ is the mid-point of $BD$. The plane is at $45^\circ$ to the horizontal, $BC$ is along a line of greatest slope of the plane and $DE$ is horizontal. The prism weighs $21\,\text{N}$, and equilibrium is maintained by a horizontal force of magnitude $P\,\text{N}$ acting along $ED$.