Two fixed equal positive charges, each of magnitude  5x10^-5C are located at points A and B, separated by a distance of 6 m. An equal and opposite charge moves towards them along the line COD, the perpendicular bisector of the line AB. The moving charge, when reaches the point C at a distance of 4 m from O, has a kinetic energy of 4 joules. Calculate the distance of the farthest point D which the negative charge will reach before returning towards C.

I am getting 8.5m app.

Using the energy equation.

0 = Delta KE + Delta PE

0 = 4 + (2KQq/root(x^2 +36) - 2KQq/5)