Final answer:
Using Newton's second law, we found the net force to be -10N to the left, and with a mass of 6kg, the acceleration would be -1.67 m/s² to the left. However, this does not match any of the provided options, so we can't confirm a correct answer based on the given information.
Step-by-step explanation:
To find the acceleration of the box, we must apply Newton's second law, which states that the force acting on an object is equal to the mass of the object multiplied by its acceleration (F = ma). First, we calculate the net force acting on the box by summing up all the applied forces. Since F1 and F2 are both to the left, we can add their magnitudes together to get a total force to the left. The applied force Fa is to the right, so we subtract it from the total force to the left to find the net force.
Fnet = F1 + F2 - Fa = 4N + 21N - 35N = -10N. The negative sign indicates that the net force is to the left. Then using this net force, we can calculate the acceleration with the formula:
a = Fnet / m = (-10 N) / (6 kg) = -1.67 m/s².
This answer does not match any of the options provided. Therefore, we cannot confirm a correct answer from the options A through D given in the question. However, if the calculation is correct, the box would be accelerating to the left since the net force is negative.