The sides of similar triangles are proportional, so the ratios of corresponding sides are equal. Corresponding sides will be located opposite corresponding (congruent) angles. For example, the measure of $\displaystyle\angle{Z}\usepackage{cmbright}\Huge\$ equals the measure of $\displaystyle\angle{C}\usepackage{cmbright}\Huge\$ , so YX corresponds to AB, and these two lengths are used in the first ratio.