To solve this, we want to have the isolate the variable x on the left side, by itself without a coefficient. All we have to do to do this is add 4 to both sides, cancelling out the -4 and getting x<13
The proportion is 1 cup of sugar for 1 dozen cookies (1/1). So if you multiply the denominator by 6 you must multiply the numerator by 6, and 6*1=6. Therefore you need 6 cups of sugar to make 6 dozen cookies.