How do you simplify (6-sqrt(20))/2?

$3 - \sqrt{5}$
Since $\sqrt{20} = \sqrt{4 \cdot 5} = \sqrt{4} \cdot \sqrt{5} = 2 \sqrt{5}$, we can write
$\frac{6 - \sqrt{20}}{2} = \frac{6 - 2 \sqrt{5}}{2} = \frac{\cancel{2} \left(3 - \sqrt{5}\right)}{\cancel{2}} = 3 - \sqrt{5}$