I keep getting these wrong and it won't tell me why or to figure it out. I need help.

The number of cars sold weekly by a new automobile dealership grows according to a linear growth model. The first week the dealership sold eight cars (Po = 8). The second week the dealership sold cars (P1 = 13). Write the recursive formula for the number of cars sold, Pn, in the (n + 1)th week. Pn = Pn-1+ Write the explicit formula for the number of cars sold, Pn, in the (n + 1)th week. Pn = If this trend continues, how many cars will be sold in the fourth week? cars