help with part b please!

(a) Show that (b) 1 mole of Van der Waals gas is expanded at constant temperature from pressure and molar volume (P1,V1) to pressure and molar volume (P2,V2). Using the result of par(a),) show that the enthalpy change for this process is given by H=a(1/V21/V1)+P2V2P1V1