I know some equation
To find the time of flight, determine the time the projectile takes to reach maximum height. The time of flight is just double the maximum-height time.Start with the equation:vy = voy + ay tAt maximum height, vy = 0.The time to reach maximum height is t1/2 = - voy / ayTime of flight is t = 2t1/2 = - 2voy / ayPlugging in voy = vo sin(q) and ay = -g, gives:Time of flight is t = 2 vo sin(q) / gwhere g = 9.8 m/s2The time of flight is also determined solely by the initial velocity in the y direction and the acceleration due to gravity.

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I suppose it is a question in kinematics and forces, acceleration due to gravity, projectile motion.

If a particle is thrown in to the air at a projection angle Ф, with an initial speed u, it flies to  a highest point and then returns to surface of earth at some distance.

Then the time for the particle to reach the highest point = u sin Ф / g
where g is acceleration due to gravity.   we use  v = u - g t  and resolution of velocity vector along the vertical direction to solve this.

Time taken for the particle to touch ground from the highest point = u sin Ф / g

Time of flight  = 2 u sin Ф / g

If the particle is thrown vertically upwards, the n  duration of flight = 2u/g  as Ф=90 degrees