Question

Two charges are placed on the X axis, a charge of 3 micro C at x=0 and
a charge of -5 micro C at x=0.40 m . Where must a third charge q be
placed if the force it experiences is to be zero ?

Answer 1

Charge 'q' can't be placed between the two charges, because net force on charge 'q' won't be zero. For the same reason, it can't be placed on other side of -5 microcoulomb charge.

Hope this helps...