\[\frac{a - 2\sqrt{3a} + 3}{a - 3} = \frac{\left( \sqrt{a} - \sqrt{3} \right)^{2}}{\left( \sqrt{a} - \sqrt{3} \right)\left( \sqrt{a} + \sqrt{3} \right)} =\]
\[= \frac{\sqrt{a} - \sqrt{3}}{\sqrt{a} + \sqrt{3}}\]