You may have thought that the $\displaystyle\sqrt{{{a}}}\usepackage{cmbright}\Huge\$ in the top and the bottom would simplify to $\displaystyle-{1}\usepackage{cmbright}\Huge\$ . That kind of simplification would hold only if $\displaystyle\sqrt{{{a}}}\usepackage{cmbright}\Huge\$ were a factor of the denominator. Multiply the numerator and denominator by the conjugate of the denominator:

$\displaystyle\frac{\sqrt{{a}}}{{{6}-\sqrt{{a}}}}\cdot\frac{{{6}+\sqrt{{a}}}}{{{6}+\sqrt{{a}}}}\usepackage{cmbright}\Huge\$

Use FOIL. Simplify radicals.