In general, to simplify a surd, you want to "rationalize the denominator".
In this case, that would involve multiplying by $\sqrt{42}$ to get $\displaystyle\frac{3\sqrt{588}}{42}$, factoring out the $196$ from the $588$ to get $\displaystyle\frac{42\sqrt{3}}{42}$, and reducing the fraction to obtain $\sqrt{3}$.
However, one property of square roots is that they can be divided. So, you can simply divide $\sqrt{42}$ in the denominator by $\sqrt{14}$ in the numerator to get $\displaystyle \frac{3}{\sqrt{3}}$, which is simply $\sqrt{3}$.