physics question

Attached is the problem:
Two circular coils of radius R, each carrying a current I in the same sense, are
parallel with the xy-plane with their centers at (0, 0,±d/2). On the z-axis the
magnetic field is ~B
= B(z)ˆz. At z = 0, halfway between the coils, @B/@z = 0.

(i) Determine d such that @2B/@z2 = 0 at z = 0 on the z-axis.
(ii) Show that for this configuration, the third derivative of the field with respect
to z is also zero at z = 0.
(iii) Use a computer to plot the magnetic field B(z) as a function of position z
along the axis.