A recursive formula is stating what the first value is, and how to get to the next number given the previous number.
Since the first number is 3, our first part is a₁=3 (if a is the function). Next, to get to -21, we multiply 3 by -21/-3=7. Therefore, aₓ(if x is the nth term of the function)=7*a₍ₓ₋₁₎ due to that we multiply 7 by the previous number