If a ball is thrown off a 17.9m roof with the velocity of 23.9m/s and it takes 1.91sec to reach the ground, What is the horizontal displacement and the Velocity right before it hits the ground?

Answer 1

They don't tell you the angle (measured above horizontal) that the ball was thrown, but you can figure it out.

For a ball thrown horizontally, the distance fallen (vertically) is
Y = (g/2) t^2 = 17.9 m

Since that is also the height of the building, it WAS thrown horizontally. The horizontal velcoity component remains 23.9 m/s

Just multiply the initial velocity by the time of flight, to get the horizontal displacement.