Answer:
Magnetic field will be equal to 
Explanation:
We have given velocity of proton 
Magnetic force experienced by proton 
Charge on proton 
Angle between field and velocity 
Force in magnetic field is equal to 
So 

So magnetic field will be equal to 
From the definition of entropy, the entropy change of an object is

where
Q is the heat absorbed
T is the absolute temperature
in our problem, we have Q=25.0 J, while the absolute temperature is (converting in Kelvin)

and so the entropy change is