The figure shows four situations in which charges as indicated (q > 0) are fixed on an axis. How many situations is there a point to the left of the charges where an electron would be in equilibrium?

For (1) Let the electron be held at a distance x from -q charge.

For equilibrium $\frac{q(-e)}{4\pi {\in }_0{x}^2}=\frac{(-e)(-4q)}{4\pi {\in }_0{(x+d)}^2}$

We can find value of x for which $F_{net}=0$  which means that electron will be in equilibrium.

For (2): for equilibrium $\frac{(-e)(-q)}{4\pi {\in }_0{x}^2}=\frac{(-e)4q}{4\pi {\in }_0{(x+d)}^2}$

 We can find value of x for which$F_{net}=0$ which means that electron will be in equilibrium.

In case (3) and (4) the electron will not remain at rest, since it experiences a net non-zero force.