How do I use summation notation to write the series 2.2 + 8.8?

I would write: S_2=((2.2(1-4^2))/(1-4)))
from the explicit formula ${S}_{n} = \left(\frac{{a}_{1} \left(1 - {r}^{n}\right)}{1 - r}\right)$. In the formula, ${a}_{1}$ represents the first term, r stands for the ratio between terms, and ${S}_{n}$ means "the sum of n terms".