Estimate the Slope of a Line?

The slope of a line is a measure of how steep it is. It is defined as

y₂- y₁/x₂-x₁

Where (x₁, y₁) and (x₂, y₂) are any two points on the line. (It doesn't matter which two points you choose. Can you figure out why?)
Horizontal lines have zero slope, because the numerator of the fraction is zero. The steeper the line, the larger the slope. Lines which slant "upward" (progressing upward from left to right) have positive slope, and lines which slant "downward" (progressing downward from left to right) have negative slope.

The slope of a vertical line is undefined, because the denominator of the fraction would be zero (which is not allowed). We often speak of a vertical line as having an "infinite" slope. This is a useful way of thinking about the vertical line, but it is not technically correct. Infinity is not a number, and so it can't be part of an equation like "slope = infinity."