You are trying to convert mass to volume. That ain't working
The spring has a spring constant of 1.00 * 10^3 N/m and the mass has been displaced 20.0 cm then the restoring force is 20000 N/m.
Explanation:
When a spring is stretched or compressed its length changes by an amount x from its equilibrium length then the restoring force is exerted.
spring constant is k = 1.00 * 10^3 N/m
mass is x = 20.0 cm
According to Hooke's law, To find restoring force,
F = - kx
= - 1.00 *10 ^3 * 20.0
F = 20000 N/m
Thus, the spring has a spring constant of 1.00 * 10^3 N/m and the mass has been displaced 20.0 cm then the restoring force is 20000 N/m.
(a) The force exerted by the electric field on the plastic sphere is equal to

where

is the charge of the sphere and E is the strength of the electric field. This force should balance the weight of the sphere:

where m is the mass of the sphere and g is the gravitational acceleration.
Since the two forces must be equal, we have:

and so we find the intensity of the electric field

(b) Now let's find the direction of the field. The electric force must balance the weight of the sphere, which is directed downward, so the electric force should be directed upward. Since the charge is negative, the force is opposite to the electric field direction, and so the direction of the electric field is downward.