Applications of lines include the use of equations or graphs of lines to model realworld situations and natural phenomena in order to make predictions and solve problems. Situations with a constant rate of change can be described by linear equations. A linear equation can be written in slopeintercept form as $y=mx+b$, where $m$ is the slope, or constant rate of change, and $b$ is the $y$intercept. The equation is useful for graphing the line and for interpreting the meaning of the slope and $y$intercept in the context of a realworld application. A more simplified form of the slopeintercept form is $y=kx$, where the $y$intercept is zero. This is the general form of a direct variation relationship, which models situations that vary proportionally.
At A Glance
 The slope of a line represents the rate of change of the dependent variable with respect to the independent variable.

Slope can be interpreted in many realworld relationships, such as speed, unit cost, and rates of change.
 Two variables vary directly if they are related by an equation of the form $y = kx$, where $k$ is a real number known as the constant of proportionality.

Direct variation can be used to model realworld situations.