We have no information describing the direction in which
the object is moving, so we're in no position to discuss its
velocity. ==> We have only speed to work with.
Kinetic energy = (1/2) (mass) (speed²)
(350 J) = (1/2) (6 kg) (speed²)
Divide each side by 3 kg : (350J / 3 kg) = (speed²)
Take the square root of each side: Speed = 10.8 m/s (rounded)
Units (for the curious/inquisitive):
[Energy] = [work] = [force] x [distance]
= [mass-distance/time²] x [distance]
= [ kg-meter / sec² ] x [ meter ]
= [ kg-meter² / sec² ] = Joule
Energy / mass = Joule/kg = [ meter² / sec² ] = (speed²)