Identify Ratios of Corresponding Sides of Similar Triangles

To understand how to find x in similar triangles, you must understand what ratios and proportions are. 
A ratio is the comparison of two quantities. 
A proportion is an equation with a ratio on one side and an equal ratio on the other. 


When two triangles are similar, they have the same angles and their sides will be in the same proportion.

For example, 

and that's x, the missing side. (: