Two point charges of magnitude +7.00 μc and -9.00 μc are placed along the x-axis at x = 0 cm and x = 40.0 cm, respectively. where must a third charge, q, be placed along the x-axis so that it does not experience any net force because of the other two charges?

third charge must be placed at x = -298.11 cm

Explanation:

When two opposite charges are placed in a straight line then the electric field is always zero at a point near to the charge of lesser magnitude

so here let say the point where electric field is zero lie on the left side of 7.00 micro C charge at a distance "x" from it

so we will have net electric field due to both charges will be zero at that point

so electric field of charge 1 must be equal to the electric field of charge 2

so we have

now square root both sides

so third charge must be placed at x = -298.11 cm

cant without the picture! ?