Lets say that you flipped the smaller triangle to be facing the same way as the bigger triangle. To find the X side, we need to do 4.5/1.2 = 1.75. which is the difference between the sides. So then we do 1.75 x 0.8 = 3.
Sorry if I couldn't explain well enough, which is not something I'm good at. I hope you get the question right! Glad I could help! :)
