A swimming-pool worker wants the pool's pH to be 7.2. She comes by to test the water and finds that the pH is 9.2. What should she do to correct this situation?

To lower the pH from 9.2 to 7.2, the worker should add an acid, such as muriatic acid or pH decreaser, to the pool water. She should follow the manufacturer's instructions on the product to gradually lower the pH level, frequently testing the water until it reaches the desired pH of 7.2.