The graph of a linear equation is a line. Linear equations can be written in several different forms, including slopeintercept form, $y = mx + b$, pointslope form, $yy_1=m(xx_1)$, and standard form, $Ax + By = C$. Vertical lines have the form $x = a$, and horizontal lines have the form $y = b$. Parallel lines have the same slope, and perpendicular lines have slopes with a product of –1. Lines can be graphed by plotting points or by using key features identified in their equations. The equation of a line can be written from a graph by using the coordinates of points located on the graph.
At A Glance
 A linear equation can be graphed by using a table to generate a list of ordered pairs. When all the ordered pairs are plotted, they lie on the same line.
 The points where a line crosses the $x$ or $y$axis are called the intercepts. To find the $x$intercept, let $y = 0$. To find the $y$intercept, let $x = 0$.
 The steepness of a line can be described by the slope. The slope of a line is the ratio of the change in $y$values to the change in $x$values of any two points on the line.
 The equation of a nonvertical line with slope $m$ and $y$intercept $b$ can be written in the slopeintercept form $y = mx + b$.
 The equation of a nonvertical line with slope $m$ that passes through the point $(x_1, y_1)$ can be written in the pointslope form $yy_1=m(xx_1)$.
 The standard form of a line is $Ax + By = C$. It is typically used for determining the $x$ and $y$intercepts of a line.
 Equations of vertical and horizontal lines are based on an undefined or zero slope. Vertical lines have the form $x = a$, and horizontal lines have the form $y = b$, where $a$ and $b$ are real numbers.

Parallel lines have the same slope but different $y$intercepts.

Perpendicular lines have slopes whose product is –1.