Ooh ok these are fun. So these triangles are similar, meaning they are in proportion. In triangle LMN, line LM is 21. To find the corresponding line on the other triangle, we need to find the corresponding letters in the names. Because L and M are the first two letters, we need to use the first two letter s in the other name, so FG. Line FG is 9, so our first proportion is 9/21
