Chapter 7 - Section 7.4 - Adding, Subtracting, and Multiplying Radical Expressions - Exercise Set - Page 439: 83

$\dfrac{-2+\sqrt{3}}{3}$

Factoring the expressions and then cancelling the common factors between the numerator and the denominator result to \begin{array}{l} \dfrac{-4+2\sqrt{3}}{6} \\\\= \dfrac{2(-2+\sqrt{3})}{2\cdot3} &\text{...cancel $2$} \\\\= \dfrac{-2+\sqrt{3}}{3} .\end{array}