
Answer:
 t = √(2d/a)
Step-by-step explanation:
To solve for a variable, undo the operations done to that variable, in reverse order. Here, t is squared, then multiplied by a/2.
 d = (a/2)t^2
Multiply by 2/a.
 2d/a = t^2
Take the square root.
 t = √(2d/a)