Answer:
The common difference between each term in the sequence is 3. To find the 21st term, we can use the formula for the nth term of an arithmetic sequence:
a_n = a_1 + (n - 1)d
where a_1 is the first term, d is the common difference, and n is the term number we want to find.
Substituting the given values, we get:
a_21 = 15 + (21 - 1)3
a_21 = 15 + 60
a_21 = 75
Therefore, the answer is (c) a^21=75.