Here is a useful worksheet on tuning forks. There is a lab version of this worksheet but if you don't have time or equipment, this is a good summary. I've also used it as a quiz at time. It relates the resonance points to the air column length and wavelength. As you know the air column length of a closed resonator is 1/4 of the wavelength. Good Luck with it. Thanks for trying Scorton Creek. It is an early page of a much thicker Physical Science workbook coming out in June 2017 by Scorton Creek.