EXAMPLE $3\mathrm{.}12$ Four vertical forces with parallel lines of action acting on a horizontal beam are shown in Figure $3\mathrm{.}28.$ Calculate the resultant moment with respect to $($a$)$ point $\backslash ($ O $\backslash ),($b$)$ point $\backslash ($ B $\backslash ),$ and $($c$)$ point $\backslash ($ C $\backslash ).$