Optimal Angle for a Fireman Shooting Water from a 20m High Roof

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A fireman shoots water out of a hose at 12m/s. he is standing on a 20m high roof and is aiming for the top of another roof at the same height, but it is 21 m away. What angle should the fireman aim the hose above the horizontal in order to hit the other roof?

i set up a chart but have no idea what to do now:
x y
vi " '
vf " -'
a 0 -9.8
d 21 0
t
 
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You need to split up the trajectory into horizontal and vertical components, start by writing down the following;

  • Initial Horizontal Velocity*
  • Initial Vertical Velocity*
  • Change in Vertical Displacement
  • Change in Horizontal Displacement
  • Acceleration in the vertical plane

Note that (*) will be trigonometric functions of the launch angle. Once you have done this, apply kinematic equations to each component independently.
 
In questions about projectiles, it is quite often, like in this case that you will need to find the time by resolving it vertically.