please explain, Engineering Statics problem.

$3$D Equilibrium: Three forces, vec$(F_1),$vec$(F_2)$ and vec$(F_3),$ act at the origin.

Determine the magnitude and the orientation $($cosine directional angles, $,,,$ in degrees$)$ of force vec$(F_2)$ such that the resultant of the three forces, vec$(F_R),$ has a magnitude of zero $($i$.$e$.,$ the system is in equilibrium$).$

b$.$ Find the angle, ${}^{\text{'}},$ between vec$(F_3)$ and vec$(F_1).$ explain in detail please