Motion with constant acceleration

It dependeds on what information you have.

If you are looking for displacement and you have vi, vf and change in time, use the first formula.

If you are looking for displacement and you have change in time and acceleration, use the third formula.

If you are looking for final velocity and you have vi, acceleration, and change in time, use the second formula.

If you looking for final velocity but you have vi, acceleration, and displacement, use the fourth formula.

 $\left(1\right)\ \ \ \ \ \Delta x=\ \frac{1}{2\ }vf\Delta t$  

 $\left(2\right)\ \ \ \ \ \ \ \ \ \ \ \ \ \ vf=a\Delta t$  

 $\left(3\right)\ \ \ \ \ \ \Delta x=\frac{1}{2}a\left(\Delta t\right)^2$  

 $\left(4\right)\ \ \ \ \ \ \ \ \ \ \ vf^2=2a\Delta x$