Force F $=(-7\mathrm{.}8$ N$)$i$+(4\mathrm{.}6$N$)$j acts on a particle with position vector r $=(3\mathrm{.}4$ m$)$i $+(5\mathrm{.}8$ m$)$j$.$ What are $($a$)$ the magnitude of the torque on the particle about the origin and $($b$)$ the angle between the directions of r and F $?$