Answer
See answer.
Work Step by Step
Using 7 of the resistors in series to make a $7.0\Omega$ resistor, then another 11 in series to make a $11.0\Omega$ resistor, will divide the voltage as required. See diagram.
The equivalent resistance is $18.0\Omega$. The current is $\frac{9V}{18\Omega}=0.5A$. The voltage between points a and b is given by Ohm’s Law.
$$V=IR=(0.5A)(7.0\Omega)=3.5V$$
