aritmetic has a common difference
-27 to -12 is a difference of 15
-12 to 3 is a difference of 15
an=a1+d(n-1)
a1=first term
d=common difference
fist term is -27
common difference is 15
an=-27+15(n-1)
84th term is n=84
a84=-27+15(84-1)
a84=-27+15(83)
a84=-27+1245
a84=1218
the 84th term is 1218
