We have the next sequence:
28,21,14,7....
We can find the common difference :
28
28-7= 21
21-7 = 14
14-7=7
Hence, the common difference is d=-7.
The recursive formula is given by:
[tex]\begin{gathered} a_n=a_{n-1}+d \\ \text{Then:} \\ a_n=a_{n-1}-7 \end{gathered}[/tex]
