How to figure out a equation given the variables

[Kennyh]

Homework Statement

Starting from the constant-acceleration kinematic equations, write a formula that gives vf in terms of t, xi, xf, and a.

Homework Equations

The Attempt at a Solution

I tried using vf = vi + at but it was incorrect. Is there another way where i can solve for vf using the 4 needed variable above? thanks
Also I have tried using this : (xf-xi)+(1/2at) = vi then substitute vi with this equation vf = vi+at. After that I got (xf-xi)+at = vf but it was still incorrect.

 

[rl.bhat]

(xf-xi)+(1/2at) = vi
This equation is wrong.
It should be
(xf-xi)-(1/2at^2) = vi*t
From the equation vf = vi + at find vi and put it in the above equation and rearrange to get vf

 

[Kennyh]

what I got was ((xf-xi)-(1/2at^2)/t))+at = vf

 

[rl.bhat]

what I got was ((xf-xi)-(1/2at^2)/t))+at = vf

vi*t = (vf - at)t = vf*t -a*t^2