Equation of scaler potential problem

[tomalku]

Homework Statement

Homework Equations

Equations of Lienard Wiechert Potential

The Attempt at a Solution

I had used the equation of scaler potential. I don't understand how to use angle 60 degree is this equation. suggestion needed

 

[Let'sthink]

I think the statement of problem is not complete. r and θ give you the coordinates of the electron as x = r cos θ and y = r sin θ; as β = 0.5 means electron is not given to be at rest.In that case we should know the relative orientation of velocity vector of electron relative to position vector or i and j.