The answer is:
L == (4*V*h - E*d**(2))/(2*E*d)
1.Derive expression for a between plates
2.Derive the angle leaving the plates using vertical and horizontal suvat
3.Use trig to get expression for h and L (remember the droplet has already travelled some vertical distance, sv between the plates)
4.Use suvat again to get the vertical distance travelled between the plates, sv
5.Substitute in and rearrange for L