Answer:
21, 18, 15
Step-by-step explanation:
First term = a1
Second term = a2
Third term = a3
a1 = 21
a2
= a + d
= 21 - 3
= 18
a3
= a + 2d
= 21 + 2 ( - 3 )
= 21 - 6
= 15
The first 3 terms are 21, 18, 15.
