Step-by-step explanation:
It is given that,
Speed of proton, 

Acceleration of the proton, 

The force acting on the proton is balanced by the magnetic force. So, 


m is the mass of proton

B = 0.020875 
or
B = 0.021 T
So, the magnitude of magnetic field is 0.021 T. As the acceleration in +x direction, velocity in +z direction. So, using right hand rule, the magnitude of B must be in -y direction.