Question

I'm not sure what this is exactly?

Answer 1

$_nC_k=_nC_{n-k}$

so
$_{100}C_{98}=_{100}C_{100-98}=_{100}C_2=4,950$

Answer 2

$\bf _nC_r=\cfrac{n!}{r!(n-r)!}\\\\\\ _{100}C_2=\cfrac{100!}{2!(100-2)!}\qquad\qquad _{100}C_{98}=\cfrac{100!}{98!(100-98)!}$