There is no external force so apply conservation of mechanical energy between points a and b
initially particle is at rest so kinetic energy at point a is zero
(KE+PE)a=(KE+PE)b
0+k(3×10−9)q00.01−k(3×10−9)q00.02=12mv2+k(3×10−9)q00.02−k(3×10−9)qo0.01
Put the values and get v=12√15=46ms−1