Two point charges $\;q_{1}\;and\;q_{2}\;$ are placed on $\;x=-a\;and\;x=+a\;respectively\;.$ Find the relation between $\;q_{1}\;and\;q_{2}\;$ so that the net electric field at $\;x=+\large\frac{a}{2}\;$ is zero

Question

$(a)\;q_{1}=q_{2}\qquad(b)\;q_{1}=9 q_{2}\qquad(c)\;q_{2}=9 q_{1}\qquad(d)\;none$