kick a ball of the roof of your house.It rolls off at a velocity of 12m/s.The roof is 21m high.How long does it take the ball to hit ground?

Take g = 9.81 m/s², and take all downward quantities to be positive.
The horizontal initial velocity is 12 m/s, but the vertical initial velocity is 0.

Consider the vertical motion (uniform acceleration motion) :
Initial velocity, u = 0 m/s
Acceleration, a = 9.81 m/s²
Displacement, s = 21 m

s = ut + (1/2)at²
21 = 0 + (1/2) × 9.81 × t²
t = √(21 × 2 / 9.81) s
Time taken, t = 2.1 s (to 2 sig. fig.)