Suppose we can change the location of the charge -2q on the x axis. The charge +q

Question:

Suppose we can change the location of the charge -2q on the x axis. The charge +q (where q = 4.11 × 10-9 C) is still at the origin.
(a) Where should the charge -2q be placed to ensure that the electric potential vanishes at x = 0.500 m?
(b) With the location of -2q found in part (a), where does the electric potential pass through zero in the region x < 0?