\[3;6;\ldots\]
\[b_{1} = 3;\ \ b_{2} = 6;\ \ q = \frac{6}{3} = 2.\]
\[S_{6} = \frac{3 \cdot \left( 2^{6} - 1 \right)}{2 - 1} =\]
\[= 3 \cdot (64 - 1) = 3 \cdot 63 = 189.\]