(Solved):  Two charges of 3.0 microcoulombs are located at the points in the plane (0, 2m) and (0, -2m), ...

 Two charges of 3.0 microcoulombs are located at the points in the plane (0, 2m) and (0, -2m), respectively. Two other charges of equal magnitude Q are located at (4m, 2m) and (4m, -2m), respectively. Knowing that the electric field vector at the origin (0, 0) is (4.0x103 N/C)i, determine the sign and value of the charges Q.