Answer:
This is based on one of the fundamental laws of algebra:
Consider:
(2 + 4 + 6) = (2 *1 + 2 * 2 + 2 * 3)
One can quickly see that
(2 *1 + 2 * 2 + 2 * 3) = 2 * (1 + 2 + 3)
The common factor is merely brought outside of the combination that contains the common factor
