Answer
$e = + \sqrt {\frac{2rE}{k}},- \sqrt {\frac{2rE}{k}} $
Work Step by Step
$E = \frac{e^2k}{2r}$
$2rE = e^2k$
$\frac{2rE}{k} = e^2$
$e = + \sqrt {\frac{2rE}{k}},- \sqrt {\frac{2rE}{k}} $
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