Walker, Halliday & Resnick — Force and Motion-I: Problem 7

7 There are two forces on the 2.00 kg box in the overhead view of Fig. 5-21, but only one is shown. For $F_1 = 20.0 \text{ N}$, $a = 12.0 \text{ m/s}^2$, and $\theta = 30.0^\circ$, find the second force (a) in unit-vector notation and as (b) a magnitude and (c) an angle relative to the positive direction of the $x$ axis.