Calculating Charge Interactions: Half-Speed & Beyond

[longcatislong]

Homework Statement

A fixed charge Q= +3.65 µC is held fixed at the origin of an xy plane. A second charge q= +5.11 µC is released from rest at the xy coordinate of (+1.15 m, +0.810 m).

a) (a) If the mass of q is 2.90 g, what is its speed when it moves infinitely far from the origin?

(b) At what distance from the origin does q attain half the speed it will have at infinity*?
(*Please note where most of the energy exchange occurs!)

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Homework Equations

Ei=Ef
KQq/r=Fe=qV
1/2mv^2

The Attempt at a Solution

I solved for a) and got 9.07202 m/s
Can't seem to figure out way to do b. Initially I just took 1/2 the velocity I got in a), plugged it into the equation KQq/r=1/2mv^2 to solve for a n R, but that's wrong. Any help is appreciated.

 

[Spinnor]

What did you get?