What is the general rule for the arithmetic sequence? 15, 12, 9, 6, … a. 3n – 18 b. 6(3 – n) c. 3n + 12 d. 18 – 3n

A1 = first term
a1 = 15

d = common difference 
d = (second term) - (first term)
d = a2 - a1
d = 9-12
d = -3
The negative common difference indicates that the terms are decreasing
Each time we subtract 3, or add -3, to get the next term.

nth term of arithmetic sequence
an = a1 + d(n-1)
an = 15 + (-3)(n-1)
an = 15- 3n + 3
an = 18 - 3n

Answer is choice D