The third floor of a house is 8m above street level. how much work must a pulley system do to lift a 150-kg oven at a constant speed to the third floor?

Potential energy of an object is

(object's mass) x (gravity) x (object's height).

When the oven is 8m above the street, it has

(150 kg) x (9.81 m/s²) x (8 m) = 11,772 kg-m²/s² = 11,772 joules

more potential energy than it has when it's on the street.

That energy has to come from somewhere, so that's

how much work must be done by somebody or something

to raise the oven up there.