use the arithmetic sequence 12 15 18 21 which is an equation for the nth term of the sequence
step 1
Find the common difference d
we have
a1=12 ------> first term
a2=15
a3=18
a4=21
so
a2-a1=15-12=3
a3-a2-18-15=3
a4-a3=21-18=3
therefore
the value of d=3
step 2
The general equation for the arithmetic sequence is
an=a1+d(n-1)
we have
a1=12
d=3
substitute
an=12+3(n-1)
an=12+3n-3
an=3n+9