Angles that add together to make a straight line are called supplementary angles. If you know one supplementary angle you can subtract it from 180° to find a missing angle.

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 ...