Understanding how to substitute values into an expression is an essential skill when using graphs to estimate values of \(x\) and \(y\). There are no straight-line segments on a quadratic graph.

If an equation can be rearranged into the form \(y = mx + c\), then its graph will be a straight line. In the above: \(x + y = 3\) can be rearranged as \(y = 3 - x\) (which can be re-written as ...