Still not getting getting correct solution

[gmuniz]

Two protons are aimed directly toward each other by a cyclotron accelerator with speeds of 1650 km/s,measured relative to the earth. find the maximum force that these protons will exert on each other.
KE_i + U_i = KE_f + U_f ;where U_i=0 and KE_f=0,so KE_i=U_f
if r= (q^2/(4*pi*epsilon_0))(1/KE_i) then F=(1/(4*pi*epsilon_0))(q^2/r^2) but the solution comes out as incorrect.

 

[Fermat]

Can you explain what this eqn is about - where does it come from?
What is the question ?
What is KE_i ?
What is F ?