Answer
F2 = (-60N) i
Work Step by Step
The box is 2kg. We are shown F1 is acting towards the right along the x axis and that it has a magnitude of 20N. In part a, we are told that ax = -20$m/s^2$. Since F = ma then the Force = 2kg * -20 $m/s^2$ = -40N. Since F1 is already 20N, then F2 must be -60N since F1 + F2 = -40N
