If the magnitude of f1 is greater than the magnitude of f2, then the box is
a) accelerating in the direction of f1
b) moving at constant speed in the direction of f1
c) moving at constant speed in the direction of f2
d) accelerating in the direction of f2

Assuming that the vectors are acting along the same axis, we could just simply add or subtract the vectors. Since the F1 is greater than F2, there would be motion, there would be acceleration, and that the direction of motion is along the F1.

what was the acceleration of the 2 kg cart?

⇒ 16.0 cm/s2

what was the acceleration of the 4 kg cart?

⇒ 8.1 cm/s2

what was the acceleration of the 6 kg cart?

⇒ 5.3 cm/s2