Field from two charges **
1 + E ≈ 1 - E/2 + 3E2/8.
A charge 2q is at the origin, and a charge -q is at x = a on the x axis.
(a) Find the point on the x axis where the electric field is zero.
(b) Consider the vertical line passing through the charge -q, that is, the line given by x = a. Locate, at least approximately, a point on this line where the electric field is parallel to the x axis.