The question is basically asking you to find an expression for the magnetic field of an infinite soleniod, through the center of the solenoid(lengthways), and around the coils of the soleniod. I know, with help from te textbook, that the magnetic field of a loop is 2(pi)b^2I/cr^3. Where b is the radius of the loop, and R is the distance to a point on the z-axis from the edge of the loop. And at the centre of the look, z=0, we have 2(pi)I/cb. Now, we also have that the infinite soleniod magnetic field is 4(pi)In/c, where n is the total number of wraps if the wire. Is this expression for in the direction of the wraps(around its centre), for the length of the cylindrical soleniod, in that direction, or both? If it is both, how would you seperate the expressions?