$= \frac{{{a^2} - {y^2}}}{{{a^2}}} $
$= 1 - \frac{{{y^2}}}{{{a^2}}}$
So, $\frac{{\left( {\frac{{3E}}{4}} \right)}}{E} = 1 - \frac{{{y^2}}}{{{a^2}}}$
$\Rightarrow \frac{{{y^2}}}{{{a^2}}} = 1 - \frac{3}{4} = \frac{1}{4}$
$ \Rightarrow y = \frac{a}{2}$.


