Express the Sum Using Summation Notation $$3 + 6 + 9 + \ldots + 21$$

Multiple Choice

Express the sum using summation notation. Use 1 as the lower limit of summation and i for the index of summation.
- $$3 + 6 + 9 + \ldots + 21$$

A) $\sum _ { i = 1 } ^ { 7 } 3 i ^ { 2 }$
B) $\sum _ { i = 1 } ^ { 7 } i ^ { 2 }$
C) $\sum _ { i = 1 } ^ { 7 } 3 i$
D) $\sum _ { i = 0 } ^ { 7 } 3 i$