What is the slope of a line that is parallel to a vertical line?

Determine the square root of 100.
$100=10\times 10$
Take the square root of the above value on both sides.
$\sqrt{100}=\sqrt{10\times 10}$
$\sqrt{100}=10$
Thus, the square root of 100 is 10.

Keep in mind that if one line is vertical, all subsequent parallel lines are also vertical.
For any two points ${\displaystyle (x_1,y_1)}$ and ${\displaystyle (x_2,y_2)}$ on a line
the slope is defined as ${\displaystyle \frac{y_2-y_1}{x_2-x_1}}$
BUT if the line is vertical ${\displaystyle x_1=x_2}$ for all points on the line
and therefore the definition of the slope would require dividing by zero (which is undefined).