Explanation

Step 1: Identify the first term and common difference

• First term $a = 7$

• Common difference $d=11-7=4$

Step 2: Use the formula for the n
th term of an A.P.:

$$a_n = a + (n - 1)d$$

For the 21st term:

$$a_{21} = 7 + (21 - 1) times 4 = 7 + 20 times 4 = 7 + 80 = 87$$