A copper wire of length 3 m and area of cross-section 1 mm2, passes through an arrangement of two frictionless pulleys, P1 and P2. One end of the wire is rigidly clamped and a mass of 1 kg is hanged from the other end. If the Young's modulus for copper is 10 x 1010 N/m2, then the elongation (in mm) in the wire is________. Take g = 10ms-2.