Field and Wave Electromagnetics 2nd Edition by David Cheng

Exercise 65

Two point charges, Q 1 and Q 2 , are located at (1, 2, 0) and (2, 0, 0), respectively. Find the relation between Q 1 and Q 2 such that the total force on a test charge at the point P (-1, 1, 0) will have
a) no x -component, b) no y -component.