Answer:
kQq(ln(L+2a/L+a))/2a
Explanation:
This question is a example of potential difference calculation.
firstly we have to calculate the potential at point +L which we can find by integration.
steps for finding it
Firstly take a small section of dx length on road of charge (Qdx/2a) which is at x distance from the point +L.
Then find dv due to that using v=kq1/r formula
So required function will be dv=kQdx/(2a*x) we will integrate on both side with limit x=L+2a to x=L+a.
Then multiply v by charge which we will be carrying from infinity to that point.