We have to assume that the truck was not moving
before he started pushing it.
After he did 12 J of work pushing it, the truck had
12 J of kinetic energy.
Kinetic energy = (1/2) (mass) (speed²)
12 J = (1/2) (3 kg) (speed²)
Divide each side by 1.5 kg : 12J / 1.5 kg = speed²
1 J = 1 newton-meter (12 kg-m²/sec²) / (1.5 kg) = speed²
= 1 kg-m² / sec²
(12 / 1.5) ( m²/sec²) = speed²
Speed = √(8 m²/s²)
Speed = 2.83 m/s .
We can't say anything about velocity, because we don't know
anything about the direction in which the truck is moving.