A beam of protons is directed in a straight line along the positive zz ‑direction

The magnitude is

The direction is  the positive x axis

Explanation:

From the question we are told that

   The  electric field is  E = 450 V/m in the negative y ‑direction

   The speed of the proton is   in the positive z direction

Generally the overall force acting on  the proton is mathematical represented as

Now for the beam of protons to continue un-deflected along its straight-line trajectory then  

So

=>      

Generally from unit vector cross product vector multiplication

So the direction of  B (magnetic field must be in the positive x -axis )

So

=>    

=>            

=>        

=>      

=>