I came across this question a long time ago and after struggling with it came up with this:

$ \left( \frac {6 + 1}{7 - 5} \right)$

which is

$ \left( \frac {7!}{2! * 5!} \right) = \frac {7 * 6}{2} = 21$

I came across this question a long time ago and after struggling with it came up with this:

$ \binom {6 + 1}{7 - 5}$

which is

$\frac {7!}{2!5!}= \frac {7 \times 6}{2} = 21$

Trying to hide my solution....

I came across this question a long time ago and after struggling with it came up with this:

I came across this question a long time ago and after struggling with it came up with this:

$ \left( \frac {6 + 1}{7 - 5} \right)$

which is

$ \left( \frac {7!}{2! * 5!} \right) = \frac {7 * 6}{2} = 21$