tolong lg ya jalannya

a = 3
b = 9 - 3 = 6

Un = bn + a - b
      = 6n + 3 - 6
      = 6n - 3

barisan aritmatika
3,9,15,21
suku pertama=a=3
beda=b=9-3=6
Un=a+(n-1)b
=3+(n-1).6
=3+6n-6
=6n+3-6
=6n-3