Answer
$6n$
Work Step by Step
For an arithmetic series, the sum for the finite series is given by:
$S_n=\dfrac{n(a_1+a_n)}{2}$
We know that the general formula of an arithmetic sequence is given by
$a_n= a_1+(n-1) d$
Given: $6,12,18,...$
Here, $d=6$
Thus, we have $a_n=6n$
