Griffiths electrodynamic problem 5.16 solution page 240

Problem 5.16 Two long coaxial solenoids each carry current I, but in opposite directions, as shown in Fig. 5.42. The inner solenoid (radius a)hasn1 turns per unit length, and the outer one (radius b)hasn2. Find B in each of the three regions: (i) inside the inner solenoid, (ii) between them, and (iii) outside both