just i need to know why vf2 we made it (vf2)^2

In order to calculate the acceleration of an object, we can use the formula below:
[tex]V=V_0+a\cdot t[/tex]Where V is the final velocity after t seconds, V0 is the initial velocity and a is the acceleration.
If we don't have the time, and have the distance traveled instead, we can use Torricelli's equation:
[tex]V^2=V_0^2+2\cdot a\cdot d[/tex]In this formula, both the initial and final velocities are squared.