Answer
$45$
Work Step by Step
The sum $(S_n)$ of the first $n$ terms of a geometric sequence can be calculated using the formula
$$S_n=\dfrac{a_1(1-r^n)}{1-r}; r \ne 1$$
where, $r$ is the common ratio, $r$ and can be computed as the quotient of a term and the term preceding it.
The given sequence has
$a_1=3$ and $r=2$
Plug these values in the formula above to obtain:
$S_n=\dfrac{3(1-2^n)}{1-2}$
With $n=4$ , the sum of the first four terms of the sequence is:
$$S_4=\dfrac{3(1-2^4)}{1-2}=\dfrac{3(1-16)}{-1}=\dfrac{-45}{-1}=45$$
