Answer:
Step-by-step explanation:
Key point: the vertical motion is free fall, the horizontal motion remains at uniform speed of 100 m/s (because there is no force in the horizontal direction if we neglect air resistance.)
The time of free fall can be calculated first:
h = 1/2 g t^2 => t = sqrt(2 h /g) = sqrt(2 * 300 m / 9.81 s) = 7.82 seconds.
In that time, the horizontal displacement will be
s = v_horizontal * t = 100 m/s * 7.82 s = 782 m.
So the box will land 782 m further than the point it was dropped from (and 300 m lower of course)