6C2= 6![6−2]![2!]{}_2 = \; \frac{6!}{[6-2]![2!]}2=[6−2]![2!]6!
6⋅5⋅4!4!⋅2!\; \frac{6 \cdot 5 \cdot 4!}{4! \cdot 2!}4!⋅2!6⋅5⋅4!
= 6⋅52\; \frac{6 \cdot 5}{2}26⋅5
= 15
