## Geometry: Common Core (15th Edition)

We use the combination equation since order does not matter. We know that: $_nC_r =\frac{_nP_r}{r!} = \frac{n!}{(n-r)!r!}$ $_7C_2 = \frac{7!}{(7-2)!2!}$ $\frac{ 7!}{5!2!}$ We also know that: $x! = x(x-1)(x-2)...(1)$ Thus, we have: 21