The explicit formula for a arithmetic sequence is a_n = a_1 + (n - 1)d.
We want to find a_21.
a_21 = 17 + (21 - 1)(10)
a_21 = 17 + (20)(10)
a_21 = 17 + 200
a_21 = 217
The 21st term is 217.
