Answer :
[tex]\frac{1}{2}at^2=d\ \ \ \ |multiply\ both\ sides\ by\ 2\\\\at^2=2d\ \ \ \ |divide\ both\ sides\ by\ a\\\\t^2=\frac{2d}{a}\ \ \ \ |square\ root\ both\ sides\\\\t=\sqrt\frac{2d}{a}[/tex]
[tex]d=\frac{1}{2}at^2\\
2d=at^2\\
t^2=\frac{2d}{a}\\
t=\sqrt{\frac{2d}{a}}[/tex]