22.5 (shower) = X (variable used in place of the amount of water she uses to wash clothes) - 7.5 (the amount she used in the shower is 7.5 gallons less than she uses for washing clothes)
22.5 = X - 7.5 《 This is the equation.
Solve is by adding 7.5 to both sides to cancel out the negative 7.5 and you get (X = 30). Hope this helps.