A displaced electron slab sees a restoring force from its sign. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Displacement creates a proportional field
Displacement 2 meters times gradient 3 newtons per coulomb per meter gives field 6 newtons per coulomb.
3N/(C*m)⋅2m=6N/C
Electron charge flips the force direction
Charge -1 coulomb times field 6 newtons per coulomb gives restoring force -6 newtons.