Mr. moseley’s car has a 20- gallon gas tank. it took 14 gallons of gas to fill his tank. use the equation 14 + g =20 to find the number of gallons g that he had before he filled his tank with gas

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

He had 6 gallons before he filled the tank.