14 + g = 20
You want g alone, but 14 is being added to g.
If you subtract 14 from 14 + g, you will get g alone.
You must do the same operation to both sides of an equation, so subtract 14 from both sides of the equation.
14 + g - 14 = 20 - 14
14 - 14 + g = 6
g = 6
Answer: He had 6 gallons before he filled the tank.
