Your scan is not easy for me to read. Also, the right side of your image is cut-off.
It looks like you somehow first simplified:
\(\displaystyle \frac{1}{\sqrt{3(x+h)}} - \frac{1}{\sqrt{3x}}\)
to:
\(\displaystyle \frac{\sqrt{x} - \sqrt{x+h}}{\sqrt{3x(x+h)}}\).
I got the following result, instead:
\(\displaystyle \frac{\sqrt{3}[\sqrt{x} - \sqrt{x+h}]}{3\sqrt{x(x+h)}}\)
Your idea is good to multiply the numerator and denominator by the conjugate of the numerator.
Give it another try.