A line's slope, or gradient, describes the extent of its slant. If its slope is 0, the line is completely horizontal and is parallel to the x-axis. If the line if vertical and parallel to the y-axis, its slope is infinite or undefined. The slope on the graph is a visual representation of the variable y's rate of change with respect to x. You can therefore calculate the slope by determining this rate of change from any two points on the line.
Identify the points' coordinates. For this example, imagine that the points have coordinates of (2, 8) and (4, 3).
Subtract the second point's y-coordinate from the first's: 8 - 3 = 5.
Subtract the second point's x-coordinate from the first's: 2 - 4 = -2.
Divide the difference between the y-coordinates by the difference between the x-coordinates: -2 ÷ 5 = -0.4. This is the line's slope.