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.. _equation_of_a_line_refresher:

Equation of a Line (Refresher)
==============================

.. image:: figures/equation_of_a_line.png
  :align: center
  :alt: Slope intercept form: y equals m times x plus b.

All line equations in this text will be presented in the form *y = mx + b*,
where *m* is the slope and *b* is the y-intercept. This is also called
slope-intercept form. The **y-intercept** is the y-value at the point where the
line crosses the y-axis. The :ref:`slope <interpreting_slope>` gives the steepness
of the line, and can be positive, negative, or zero. Slope-intercept form is a
method to better understand the relationship between the two variables *x* and *y*.


.. image:: figures/negative_slope.png
  :width: 49%
  :align: center
  :alt: A graph of the equation y equals negative one third times x plus five and the slope is negative.

This figure depicts an equation of a line that has a y-intercept of 5 and a negative slope.

.. image:: figures/positive_slope.png
  :width: 49%
  :align: center
  :alt: A graph of the equation of y equals 2 times x plus one.

The equation of the line in the above figure has a y-intercept of 1 and a positive slope.

For additional review on slope-intercept form you can also watch out `this video
<https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:forms-of-linear-equations/x2f8bb11595b61c86:intro-to-slope-intercept-form/v/slope-intercept-form>`_.