Chapter 30, Problem 95PQ

A proton enters a region with a uniform electric field $\stackrel{\to }{E}=5.0\hat{k}\text{V/m}$ and a uniform magnetic field $\stackrel{\to }{B}=5.0\times {10}^{-4}\hat{k}\text{T}$. The proton has initial velocity ${\stackrel{\to }{v}}_0=2.5\times {10}^5\hat{\iota }\text{m/s}$. How far along the z axis does the proton travel after it undergoes three complete revolutions?