Answer:
 a[n] = a[n-1] +(3n +2)
Step-by-step explanation:
The difference between Day 1 and Day 2 is 9 - 1 = 8 cells. The difference between a2 and a1 in each of the answer choices is a2 -a1 = ...
 3n = 3(2) = 6 ≠8
 n+2 = 2+2 = 4 ≠8
 3(n+2) = 3(2+2) = 12 ≠8
 3n +2 = 3(2) +2 = 8 . . . . . . this is the appropriate choice:
 [tex]\boxed{a_n=a_{n-1}+(3n+2)}[/tex]