Pressure is defined as the force exerted per unit of area. From the information given, the force exerted can be attributed to the mass of the load carried by the piston. This means that its weight is equivalent to the force it exerts.
Weight = mass(9.81) = 3433.5 Newtons
Area of piston = 1 m^2
Pressure = Force/Area = 3433.5/1 = 3433.5 N/m^2 or Pascals.