​ Help needed

Hey there!!

This is what i would do:
 

$t=\frac{\sqrt{2d}}{\sqrt{a}}\times\frac{\sqrt{a}}{\sqrt{a}}$

$t=\frac{\sqrt{2d}\times\sqrt{a}}{a}$

$t=\frac{\sqrt{2}\sqrt{da}}{a}$

I hope this helped :)