send to an expert pleasei attached the solution but please explain in more details, especially the part where we get the volume, i don't understand where that formula came from or why

The location of the horizontal force $($center of pressure$)$ is

$y_p=y_c+\frac{I_c}{y_{c}A}=3\mathrm{.}2+\frac{(0\mathrm{.}8{)}^4}{4(3\mathrm{.}2)(0\mathrm{.}8{)}^2}=3\mathrm{.}25m$

The vertical force is applied at a distance of $x_p=3\frac{r}{8}=0\mathrm{.}3$ from the center.

The resultant force is $64$ kN acting at an angle of $-9\mathrm{.}44$ with the horizontal.