The collective oscillation scale is a square-frequency ledger. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The frequency square is a normalized density budget
Normalized constant 12 per second squared times density ratio 3 over mass ratio 6 gives omega squared 6 per second squared.
ω2=612⋅3=6s−2
Density raises the collective oscillation scale
The result is omega squared 6 per second squared for the whole displaced electron slab.