Kendrick exercises by doing pull-ups. Each week he increases the number of pull-ups he can do by the same amount. After four weeks of training, he can do 9 pull-ups. After ten weeks of training, he can do 18 pull-ups. (a) Write an equation that relates the number of pull-ups he can do (P) to the number of weeks he has been training (W). (b) How many pull-ups could he do when he started his training? (c) After how many weeks will he be able to do 24 pull-ups?