just find when the height is zero
20.4 sin47° t - 4.9t^2 = 0
t = (20.4 sin47°)/4.9
A football is kicked from ground level with an initial velocity of 20.4 m/s at angle of 47.0° above the horizontal. How long, in seconds, is the football in the air before it hits the ground? Ignore air resistance.
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