Help appreciated!

The electric field at a location B on the z- axis and has a magnitude of 5 10^5 N/C and points in the positive r direction. You must place the following two charges to produce this field: a positive charge of 1.9 C and a negative charge of - 1.9 C and both charges must lie on the same line passing through B and be equidistant from B. What is the distance of the charges from B. Answer in units of m.