If the zero level is a distance
2h/3
above the floor, what is the potential energy
u
of the block on the floor?

Potential energy, U ?, is given by U=mgh. so, were the zero level the floor, and the block were at the distance quoted,U would be mg2h/3.but, the q says that the zero is at 2h/3.so U = -mg2h/3. Since U is a scalar quantity, this is a "signed number", it's not a vector.

Answerwavelength;