Answer
$a = 0 m/s^2$
Work Step by Step
According to second Shell Theorem, If the body is a spherically symmetric shell, there will be no net gravitational force exerted by the shell on any object inside, regardless of the object's location within the shell.
Since $r = 0.5 R$, this means the electron is inside the shell, so the net force is zero; $F = 0$
According to Newton's Second Law of motion, acceleration can be calculated by
$F = ma$
$a = \frac{F}{m}$
$a = \frac{0}{m}$
$a = 0 m/s^2$
