Gravitation due to the planet's mass is always against the ejected particle velocity. This will result in deceleration of the ejected particle and eventually bring the velocity to zero at the topmost point of the trajectory before pulling it back to the ground.
By the kinematic equation
[tex]v^2 - u^2 = 2.a.s[/tex]
where s is the distance travelled = 1.52 x 10^5 m
a is the acceleration due to gravity = -1.91 m/s^2
v and u are initial and final velocities, so v = 0.
Therefore
[tex]u = \sqrt{s.a.s}[/tex]
u = 761.99 m/s
