Please give me a detailed explanation because my professor failed to explain this to our class as we have an exam on Monday.

There are two forces on the 2.71 kg box in the overhead view of Fig. 5-31 but only one is shown. ForF1 = 18.9 N,a= 14.2 m/s2, and θ = 22.0°, find the second force (a) in unit-vector notation and as (b) a magnitude and (c) a direction. (State the direction as a negative angle measured from the +x direction.)

Fig. 5-31

Problem 5.