Final answer:
The magnitude of the horizontal force exerted on block a is equal to the magnitude of the force exerted by block b on block a.
Step-by-step explanation:
In this scenario, block a with mass m is being pushed by a horizontal force Fa. Block b, with mass 3m, exerts a force F on block a. Based on Newton's third law, the magnitude of the force exerted by block b on block a is equal to the magnitude of the force exerted by block a on block b.
Therefore, the magnitude of Fa is equal to the magnitude of F.