Smaller triangle is pre-image. The larger one is the image.
Always put the image on top when you are trying to make it into a fraction, and put the pre-image on the bottom. Since the image is 18 we put the it on top and the 8 (pre-image) on the bottom 18. 18/8. Divide it. 18 ÷ 8.
Final answer: 2.25
