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.