The relevant formula we can use in this case is:
v^2 = v0^2 + 2 a d
where,
v = final velocity = 0
v0 = initial velocity = ?
a = acceleration = -9.8 m/s^2 (negative since opposite)
d = 100 km = 100,000 m
0 = v0^2 + 2 (-9.8) (100,000)
v0 = 1400 m/s