Answer
The electric field at a radial distance of $r = 3.50 a$ is zero.
Work Step by Step
We can draw a Gaussian sphere with a radius of $r = 3.50 a$
We can find the flux through the sphere:
$\Phi = \frac{q_{enc}}{\epsilon_0}$
$\Phi = \frac{5.00~fC-5.00~fC}{\epsilon_0}$
$\Phi = 0$
Therefore, the electric field at a radial distance of $r = 3.50 a$ is zero.
