Young modulus for steel is 2*10^12n/m^2 what extension will be produced in a steel wire of 1mm in diameter and 2.5m long by hanging a mass of 3 kg from it

Remember that extensional strain = Young's Modulus times tensile stress:

\sigma = E \epsilon

For a simple bar under tension you have \sigma = Force/Area, and \epsilon= \Delta L/L_0. Solve for \Delta L.