Answer
$2049$
Work Step by Step
Formula to calculate the sum of the first $n$ terms of a geometric sequence is: $s_n=\dfrac{a(1-r^n)}{(1-r)}$
Here, $r=\dfrac{a_2}{a_1}=-2$
As we are given that, $n=11$
Thus, $s_{12}=\dfrac{3(1-(-2)^{11})}{(1+2)}$
or, $s_{11}=2049$