solve the system to find a=-7 and d = 3 ..it is easy...
Hint: if since 7 is prime that means these terms must be either -7 and -1 or +1 and +7. And if that means that one of these terms must be negative, or perhaps 0. Given these constraints, and given that as an arithmetic progression the difference between successive terms is constant and positive, there is only one sequence that fits. Work on it a bit and post back with what you get for an answer.