Answer
(a) positive x axis
(b) $F=\frac{Kq^2}{R^2}$
Work Step by Step
(a) We know that the twelve identical charges are on the circumference of a circle of radius $R$ and are equally spaced. One of theses charges lying on the +x axis is moved to the center of the circle. As the five pairs of charges are placed exactly opposite to each other, the net force exerted due to these charges on the charge placed at the center gets canceled. Thus, the net force on this charge is due to the charge on the negative x axis and it is directed towards the positive x axis.
(b) We use Coulombs' law to determine the magnitude of the net electric force acting on the charge at the center; that is, $F=K\frac{q^2}{R^2}$.