Work backwards.
s=ut+(at^2)/2
Subtract ut.
s-ut=(at^2)/2
Multiply by 2.
2(s-ut)=at^2
Divide by a.
2(s-ut)/a=t^2
Find the square root:
s=ut+(at^2)/2
Subtract ut.
s-ut=(at^2)/2
Multiply by 2.
2(s-ut)=at^2
Divide by a.
2(s-ut)/a=t^2
Find the square root: