Answer:
T1=12 a=12 a=T1
d=9
your Arithmetic equation is Tn=a+(n-1)d
Tn=a+(n-1)d
SUBSTITUTE a and d in your equation
Tn=12+(n-1)(9)
Tn=12+9n-9
Tn=3+9n or Tn=9n+3
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