Answer
$36$
Work Step by Step
RECALL:
${n}\choose{k}$$=\dfrac{n!}{k!(n-k)!}$
Use the formula above to obtain:
${9}\choose{7}$$\require{cancel}
\\=\dfrac{9!}{7!(9-7)!}
\\=\dfrac{9!}{7!2!}
\\=\dfrac{9\cdot 8 \cdot 7!}{7! (2\cdot 1)}
\\=\dfrac{9\cdot \cancel{8}^4 \cdot \cancel{7!}}{\cancel{7!} (\cancel{2}\cdot 1)}
\\=36$
