I don't understand how to do c?

4 An object slides down an inclined plane, which slopes downwards at an angle of 20 to the horizontal. The object reaches a speed of 40 m/s at the end of the slide, which is 15 m above the ground. Take the end of the slide as the origin, and let i and j be unit vectors in the forwards and upwards directions respectively. a Let r(t) be the position of the object at time 3 seconds after leaving the end of the slide. Using for) = g j, nd r(r). Hence nd, correct to one decimal place: b the horizontal distance, in metres, travelled by the object to reach the ground after it leaves the end of the slide c the angle upwards from the horizontal, in degrees, at which it hits the ground