Displaced electron charge sees a restoring force by sign convention. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Separation creates an electric field
A displacement of 2 meters with field gradient 3 newtons per coulomb per meter gives field 6 newtons per coulomb.
3N/(C*m)⋅2m=6N/C
A negative charge feels the restoring force
The electron slab charge is -1 coulomb, so the force is -6 newtons and points back.