I suppose you mean an= 2 * (1/3)^n
a2
\(=2\times(\dfrac{1}{3})^2\\ =\dfrac{2}{9}\)
Sn OR S
\(=\dfrac{a_1(1-r^n)}{1-r}\\ =\dfrac{2\times(\dfrac{1}{3})^1\times(1-\dfrac{1}{3^n})}{1-\dfrac{1}{3}}\\ =\dfrac{3^n-1}{3^n}\)
S3
\(=\dfrac{3^3-1}{3^3}\\ =\dfrac{26}{27}\)