Solution:

Given that,
The sum of GP $=728$
Where, $a=2, r=6 / 2=3, n=?$
Using the formula,
The sum of GP for $n$ terms $=a\left(r^{n}-1\right) /(r-1)$
$\begin{array}{l}
728=2\left(3^{n}-1\right) /(3-1) \\
728=2\left(3^{n}-1\right) / 2 \\
728=3^{n}-1 \\
729=3^{n} \\
3^{6}=3^{n} \\
6=n
\end{array}$
As a result, 6 terms are required to make a sum equal to 728