Meryl needs to add enough water to 11 gallons of an 18% detergent solution to make a 12% detergent solution. which equation can she use to find g, the number of gallons of water she should add?

A suitable equation will equate the quantity of detergent both before and after the water is added to the solution.

the amount of detergent in 11 gal of 18% solution is the same as the amount of detergent in the 12% solution (after g gallons of water are added)
  .18*11 = .12*(11+g)