The Graph of f

To draw a function, let $y$ be its output. The graph of $f$ is the graph of $y = f\left(x\right)$. For $f\left(x\right) = -x+2$, the graph is the line with slope -1 through (0, 2). Each point on the line pairs an input $x$ with its output $f\left(x\right)$.

To read $f\left(3\right)$ from the graph, find the point on the line where $x = 3$ and read its y-coordinate. Writing a linear rule as a function does not change how you find its slope or its intercepts.