A 1.0-m-diameter oil droplet (density 900 kg/m 3 ) is negatively charged with the addition of 25

A 1.0-μm-diameter oil droplet (density 900 kg/m³) is negatively charged with the addition of 25 extra electrons. It is released from rest 2.0 mm from a very wide plane of positive charge, after which it accelerates toward the plane and collides with a speed of 3.5 m/s. What is the surface charge density of the plane?