Two lines are perpendicular if you multiply the gradients of the two lines together and the
result is -1.
To remember this formula work out what the slopes of an X would be.
`Gradient (m) =y/x`
`Gradient\ of\ li\n\e\ A=1/1-1`
`Gradient\ of\ li\n\e\ B=1/-1-1`
This shows the connection between lines with gradients 1 and -1.
When we multiply them together `1\times-1 = -1`