==> $\omega = \frac{{{v_{\max }}}}{a} = \frac{{10}}{4}$
Now, $v = \omega \sqrt {{a^2} - {y^2}} $
==> ${v^2} = {\omega ^2}({a^2} - {y^2})$
==> ${y^2} = {a^2} - \frac{{{v^2}}}{{{\omega ^2}}}$
$y = \sqrt{{a^2} - \frac{{{v^2}}}{{{\omega ^2}}}}$
$= \sqrt{{4^2} - \frac{5^2}{{({\frac{10}{4}})^2}}}$
$ = 2\sqrt 3 \,cm$

$y = A\,\cos \,\omega t\,\cos \,2\omega t + A\,\sin \,\omega t\,\sin \,2\omega t$.
Than the nature of the function is