Answer
$y = -5.1m$
Work Step by Step
Gravitational force is $F = m_eg$ and electric force is $F = \frac{kq^2}{r^2} $. We equate these two equations and solve for $y$, which in this case is $r$
$ \frac{kq^2}{y^2} = m_eg $
$y^2 = \frac{kq^2}{m_eg} $
$y^2 = \frac{(8.99 \times 10^9 N . m^2 / C^2) (1.6 \times 10^{-19} C)^2}{(9.11 \times 10^{-31} kg) (9.8m/s^2)}$
$y^2 = 25.778355 m $
$y = \pm 5.1 m$
We choose $y = -5.1m$ here because second electron must be below the first one, so that the repulsive force is acting opposite of the Earth's gravitational pull.
