You may have just found the conjugate of the denominator. Multiply the numerator and denominator by the conjugate of the denominator:

$\displaystyle\frac{\sqrt{{a}}}{{{5}-\sqrt{{a}}}}\cdot\frac{{{5}+\sqrt{{a}}}}{{{5}+\sqrt{{a}}}}\usepackage{cmbright}\Huge\$

Use FOIL. Simplify radicals.