Question

D = vt+1/2at^2 solve for t

Answer 1

you should write it in "standard form"
a t^2 + b t + c =0

then use the "quadratic formula"
https://en.wikipedia.org/wiki/Quadratic_formula#Derivation_of_the_formula

Answer 2

the answer will be a messy expression with D, v and a in it (not a simple number)