If you are asking regarding Time of Flight for a Projectile , then here is how you can derive it's equation.

Let the velocity with which the projectile was fired = V

Let the angle which the projectile makes with Horizontal = θ (theta)

So velocity of particle in Y axis = V sin θ

Formula for the Height (Displacement) achieved by an object when thrown upwards is: S =  ut + (1/2) at^2

From the above formula, displacement (S) for a projectile in Y-axis = 0 (since it goes up and then again falls back to ground).

So, 0 = ut + (1/2) at^2

⇒ 0 = V sinθt + (1/2) at^2

⇒ t = (2V sin θ)/g

So time of flight for a projectile is: T = (2V sin θ)/g