Answer
$\left\{\text{ab, ac, ad, ae, bc, bd, be, cd, ce, de}\right\}$
$C(5,2) = 10$
Work Step by Step
Order is not important in a combination.
For example, $ab$ and $ba$ are considered as same combination.
We list all possible arrangements taken 2 at a time from $a,b,c,d$ and $e$ and exclude arrangements with same objects but only different in order ( See definition of 'combination'):
$\left\{\text{ab, ac, ad, ae, bc, bd, be, cd, ce, de}\right\}$.
Use the formula $C(n, r)=\dfrac{n!}{(n-r)!\times r!}$:
$C(5,2)
\\= \dfrac{5!}{(5-2)!2!}
\\= \dfrac{5!}{3!\times2!}
\\= \frac{5\times4\times3!}{3!\times2!}
\\ = 10$
