Answer:
4.5 m/s
Explanation:
The rock must barely clear the shelf below, this means that the horizontal distance covered must be

while the vertical distance covered must be

The rock is thrown horizontally with velocity
, so we can rewrite the horizontal distance as

where t is the time of flight. Re-arranging the equation,
(1)
The vertical distance covered instead is

where we omit the term
since the initial vertical velocity is zero. From this equation,
(2)
Equating (1) and (2), we can solve the equation to find
:
