SAT Math X

The estimated population of rabbits in a certain forest is given by the function $P(t)=at+120$ where $t$ is an integer which represents the number of years after the rabbit population was first counted, $0\le t \le 10$, and $a$ is a constant. If there were $192$ rabbits $3$ ears after the population was first counted, how many rabbits will there be $7$ years after the population was first counted?