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The slope of a line measures the deviation of the line from the horizontal. It is usually denoted by the letter m.

The slope is also called the gradient of the line, measured as the angle away from the horizontal axis.

The slope can be expressed in two ways:

From the coordinates of any two points on the line, the slope m is
$m = \frac{\Delta y}{\Delta x} = \frac{change-in-y}{change-in-x} = \frac{y_{1} - y_{2}}{x_{1} - x_{2}} = \frac{y_{2} - y_{1}}{x_{2} - x_{1}}$

In the slope-intercept form as
$y = mx + c$

Note:

The slope of this horizontal line is zero.
(because any two y points on a horizontal line have the same value, their difference is zero)

The slope of a vertical line is undefined.
(because any two points x on a vertical line have the same value, their difference is zero and divide by zero is not defined in mathematics)