## Precalculus (6th Edition) Blitzer

$21$
Binomial Coefficient can be defined as: $\displaystyle \binom{n}{k}=\dfrac{n!}{k!(n-k)!}$ Need to apply the formula to get the Binomial Coefficients. Here, $n=7,k=2$ Therefore, $\displaystyle \binom{7}{2}=\dfrac{7!}{2!(7-2)!}$ $=\dfrac{ 7 \cdot 6 \cdot 5!}{ (2 \cdot 1 )(5!)}$ $\bf{(Simplify)}$ $=21$