You may have thought that the $\displaystyle\sqrt{{{a}}}\usepackage{cmbright}\Huge\$ in the top and the bottom would simplify to 1. 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}}}{{\sqrt{{a}}-{5}}}\cdot\frac{{\sqrt{{a}}+{5}}}{{\sqrt{{a}}+{5}}}\usepackage{cmbright}\Huge\$

Use FOIL. Simplify radicals. The correct answer is: $\displaystyle\frac{{{a}+{5}\sqrt{{{a}}}}}{{{a}-{25}}}\usepackage{cmbright}\Huge\$