In your proportion you will have 3 of the 4 numbers such as 3/7=9/x. Mulitply the nubers that are diagonal (cross) from each other. Here it is the 7 and 9 so 7x9 = 63. Then you will divide that answer by the number that you haven't used yet, here that is the 3. So 63/3=21. x=21. Works all the time.