We can use the Pythagorean theorem to do this. The formula for $r$ in terms of $x$ and $y$ is $r = \sqrt{x^2 + y^2}$. Substituting $x = -3$ and $y = -\sqrt{3}$, we get $r = \sqrt{(-3)^2 + (-\sqrt{3})^2} = \sqrt{9 + 3} = \sqrt{12} = 2\sqrt{3}$.
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