Question: 2017 Fall Midterm - 9
What is the coefficient of $x^{20}y^{80}$ in the expansion of $(7x-13y)^{100}$?
(a)
${100 \choose 80} \cdot 7^{20} \cdot 13^{80}$
(b)
$- {100 \choose 80} \cdot 7^{20} \cdot 13^{80}$
(c)
$- {100 \choose 20} \cdot 7^{80} \cdot 13^{20}$
(d)
${100 \choose 20} \cdot 7^{80} \cdot 13^{20}$