The electric field strength between two parallel plates is uniform - it will have the same value at every point between the plates, so the actual position of P is irrelevant (so long as it is between the plates!).
Do you know an equation for the electric field strength between two parallel plates with a potential difference of V between them, and which are separated by a perpendicular distance d? This should help you to see why the magnitude of the electric field strength is 150 V m^-1.
As for direction - electric fields are taken by convention to "point" from positive to negative, so the direction of the field is from the plate with the larger potential to that of lower potential (taking sign into account, not just magnitude). This explains why the electric field strength - which is a vector - points to the left.