A thin current element extending between z = _ L/2 and z =L/2

A thin current element extending between z = _ L/2 and z =L/2 carries a current I along z through a circular cross section of radius a.
(a) Find A at a point P located very far from the origin (assume R is so much larger than L that point P may be considered to be at approximately the same distance from every point along the current element).
(b) Determine the corresponding H.