Answer
$\sum_{n=1}^7 3n$
Work Step by Step
The summation notation has the form of $\sum_{n=p}^q a_n$ where; $p$ is the lower limit and $q$ is the upper limit.
Recall the explicit formula for an arithmetic series.
$a_{n}=a_1+(n-1) d$
where; $a_n = n^{\text{th}} term; \\ a_1 = \ first \ term;
\\ n =\ Number \ of \ Terms ; \ \\ d =\ Common \ Difference $
From the given data series, we have:
$a_{1}=3; d=3$
Thus, substituting these values into the formula above gives:
$a_{n}=3+(n-1) \times 3=3+3n-3=3n$
Since, there are 7 terms, so $p=1$ and $q=7$
Therefore, the summation notation of the series is: $\sum_{n=1}^7 3n$
