You may have found the correct answer, but then thought that the 25 on the top and the bottom would simplify to 1. That kind of simplification would hold only if 25 were a factor of the numerator and denominator. Multiply the numerator and denominator by the conjugate of the denominator:

$\displaystyle\frac{{{5}}}{{{\left(\sqrt{{{x}}}+{5}\right)}}}\cdot\frac{{{\left(\sqrt{{{x}}}-{5}\right)}}}{{{\left(\sqrt{{{x}}}-{5}\right)}}}\usepackage{cmbright}\Huge\$

Use the Distributive Property and FOIL and simplify radicals. The correct answer is: $\displaystyle\frac{{{5}\sqrt{{{x}}}-{25}}}{{{x}-{25}}}\usepackage{cmbright}\Huge\$