If x is a side, set up a proportion. You are usually given four pieces of information all together (maybe 3 numbers and x, maybe 2 numbers and two expressions of x). Corresponding sides go in the same fractions. For example, say triangles ABC and DEF are similar. Then you know sides AB/DE = BC/EF.

If x is an angle, corresponding angles are equal. So, for triangles ABC and DEF, angleABC = angleDEF.