How to enlarge with a negative scale factor is a little less intuitive, but it’s not much more difficult. We still start by drawing lines from the centre of enlargement – here, the origin – to each corner of the shape. Now, rather than extending the lines outward from the corner, we extend the lines past the centre of the enlargement.
Because the scale factor is -1, the extension part of the lines (the part that goes outward from the origin, away from the shape) will be the same length as the original lines that were drawn from the corners to ABC. If the scale factor were -2, then the extension part of the lines would be twice the length of the original lines. This is subtly different to positive scale factors, so make sure you understand it.
For example, the line from the origin to C goes 2 to the right and 1 up. So, the extension to this line will, from the origin, go 2 to the left, and 1 down. Carrying this on with all the points, and then joining up the ends of the lines (since they form the corners of our shape), we get
If you have a keen eye, you’ll notice this is actually equivalent to rotating the shape around the centre of enlargement by 180\degree. Neat.