This equation shows that C (Noah's cousin) is equal to N (Noah's age) minus 7 years. You could test it out by plugging in any number for N. For example if Noah is 14 years old, then N=14. 14-7=7, so his cousin is 7 years old. If Noah is 30 years old, N=30. 30-7=23, so his cousin would be 23 years old.