Question

The slope of a line parallel to X-axis is ___ .

Answer 1

The slope of a line passing through points $(x_1,y_1)$ and $(x_2,y_2)$ is $\frac{y_2-y_1}{x_2-x_1}$.

Given the line is parallel to X-axis.

$⟹$ the points on the line has the same ordinate. I.e. $y_1=y_2$

Therefore, $y_2-y_1=0$.

I.e. the numerator of the expression $\frac{y_2-y_1}{x_2-x_1}$ is zero. Hence the slope of a line parallel to X-axis is zero.