its simply the poetential due the cone at its vertex multiplied by the charge q...
y/r = x/L => y= xr/L
dq = 2Πydxσ = 2Πrσxdx/l
for this ring potential at vertex = dV = ∫kdq/x
integrating from 0 - L ,
V = 2Πkrσ = Q / 2Πε0L
Now work done = Vq = Qq / 2Πε0L