The equation of a stratight line on a graph is expressed in slope-intercept form.
y = mx + b
y = y-coordinate for a point on the line.
x = x-coordinate for the same point on a line
m = slope
b = y-intecept ...where the line crosses the vertical axis
For the example graph above, the equation is
It would be tempting to look at the graph and call the y-intercept 2. The problem with this interpretation is the fact that the graph’s horizontal numbers do not start at zero. Therefore the axis at the left is not the y-axis. The y-axis is where x = 0.
Calculating the y-intercept
To calculate the y-intercept, use y = mx +b and solve for b. Pick ANY point on the line to find x and y. Calculate the slope as shown above. Plug these values into y = mx + b and solve for b.
EXAMPLE: from the graph above. x= 60, y = 10 (that’s the first point on the graph, from the data) and m = 4/25.
Therefore the correct equation for the line is
by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)