Kinetic energy of a moving object = (1/2 of the mass) x (speed²) .
Divide each side by (1/2 of the mass)
and you get:
Speed² = (KE) / (1/2 of the mass)
Take the square root
of each side: Speed = √ (2 · KE / mass) .
That's what you need, right there.
If you know the kinetic energy, then ...
-- double it,
-- divide by the object's mass,
-- take the square root.
Now you have the object's speed.