## Elementary Algebra

Horizontal lines have a slope of $0$. Slope is defined as the $\frac{\text{change in}\:y}{\text{change in}\:x}$. If a line is horizontal, that means that it's $y$ value is constant. When we go to calculate slope, we have a $0$ in the numerator, which makes the slope $0$. Therefore, any line which is horizontal will have a slope of $0$.