Problem P$4\mathrm{.}4$

A vector polygon for summing $2-$kN and $7-$kN forces is shown $($Figure P$4\mathrm{.}4).$ Determine $($a$)$ the magnitude R of the resultant by using the law of cosines and $($b$)$ its angle of action $\backslash$theta by using the law of sines.