Condition for Straight Lines in Plane to be Parallel/Slope Form

Theorem
Let $L_1$ and $L_2$ be straight lines in the Cartesian plane.

Let the slope of $L_1$ and $L_2$ be $\mu_1$ and $\mu_2$ respectively.

Then $L_1$ is parallel to $L_2$ :


 * $\mu_1 = \mu_2$