If you take a look at a map of Scotland, you'll notice an eerily straight line running through the highlands, this is the Great Glen Fault the product of half a billion years of time and geology.

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 ...