Answer:
The magnitude of the frictional force is 50 newtons.
Step-by-step explanation:
The frictional force can be found as follows:


Where:
F: is the applied force = 50 N
: is the frictional force
m: is the box's mass
a: is the acceleration
Since the box is pulled at a constant velocity, a = 0, so:


Therefore, the magnitude of the frictional force is equal to the applied force, that is to say, 50 newtons.
I hope it helps you!