In a cell if ψp = +0.3mpa and ψs=-0.45mpa, then the resulting ψ is enter your answer with either a + or a - before the number and no words.

The answer is Ψ = -0.15 MPa.

The movement of water depends on the presence of solute, so it is referred as solute potential. But, as water enters the cells, it increases a pressure inside the cell, so this pressure is called pressure potential. The water potential (Ψ) is equal to the sum of pressure potential (ΨP) and solute potential (ΨS):
Ψ = ΨP + ΨS.
ΨP = +0.3 MPa
ΨS = -0.45 Mpa
Ψ = +0.3 MPa + -0.45MPa
Ψ = 0.3-0.45 MPa
Ψ = -0.15 MPa