Answer:
B = 5.23 T
Step-by-step explanation:
Given that,
Electric field, E = 550 V/m
The speed of the proton, v = 105 m/s
We need to find the magnitude of the magnetic field such that the beam of protons continues along its straight-line trajectory.
To move in a straight line, the magnitude of the electric force from the field and the magnetic field must be equal i.e.

So, the magnitude of the magnetic field is equal to 5.23 T.