posted by Zack .
The electric field lines surrounding three charges are shown in the figure below. The center charge is q2 = -15.9 µC.
Divide the negative charge by two. Since the total charge of the dipole is zero, you must have the two positive charges (which are the same), cancel out the sole negative charge. -15.9/2 = -7.95, and then flip the sign because the two sides are positive, so +7.95µC