First find the common difference.
This can be found by subtracting the second term minus the
first term which in this case us 3 - (-6) or 3 + (+6) which is 9.
So we add 9 to reach the next term in this sequence.
Since this sequence isn't too long, continuing adding 9
until you reach the 17th term in this arithmetic sequence.
-6 ⇒ 1st term
3 ⇒ 2nd term
12 ⇒ 3rd term
21 ⇒ 4th term
30 ⇒ 5th term
39 ⇒ 6th term
48 ⇒ 7th term
57 ⇒ 8th term
66 ⇒ 9th term
75 ⇒ 10th term
84 ⇒ 11th term
93 ⇒ 12th term
102 ⇒ 13th term
111 ⇒ 14th term
120 ⇒ 15th term
129 ⇒ 16th term
138 ⇒ 17th term
By manually doing this, we found that our 17th term is 138.