You need to set up ratios to solve for x. It's all about proportions here. I'm going to set up the ratio with the smaller triangle stuff (the one on the left) on top and the larger triangle stuff on bottom. Corresponding sides rule the day.[tex] \frac{8}{12}= \frac{x}{18} [/tex]. Cross-multiply to solve for x. 12x=144 and x = 12.
