A $180^\circ$ rotation around the origin in the counter-clockwise direction is applied to $-6 - 3i.$  What is the resulting complex number?
I know that a rotation around the origin by an angle $\theta$ can be achieved by multiplying the original complex number by $e^{i\theta},$ where $i$ is the imaginary unit and $e$ is the base of the natural logarithm. So, in this case, I need to multiply $-6 - 3i$ by $e^{i\pi},$ since $\pi$ radians is equivalent to $180^\circ.$ I also know that $e^{i\pi} = -1,$ by Euler's formula. Therefore, the resulting complex number is just $(-6 - 3i) \cdot (-1) = 6 + 3i.$ 