1. The problem statement, all variables and given/known data This is for a practice question on an exam: I am able to finish the problem, if I could figure out how to find the radius of this arc the proton makes. 2. Relevant equations I have nothing. 3. The attempt at a solution I have tried arc length equations and just integrating the change in dx and dy, but all my answers have been nonsensical. I have a feeling this is very simple and I wasn't paying attention enough in Geometry back in grade school. Any help would be greatly appreciated.