The function $f(x)=x$ is the parent function of all linear functions. All other linear functions are a transformed — or changed — version of $f(x).$ To understand all lines, it is necessary to understand $f(x)=x.$ Writing this function as an equation in slope-intercept form gives Thus, the slope of this line is positive $1$ and the $y$-intercept is $(0,0).$ The graph of this line is shown.

When analyzing linear transformations, it's helpful to consider how the slope and $y$-intercept of the transformed line compare to that of $f(x)=x.$