Answer
$ a_{n}=3(4)^{n-1}$ and $ a_{7}=12288$
Work Step by Step
The general formula to find the nth term of a Geometric sequence is given as: $ a_{n}=a_1r^{n-1}$
$ r=\dfrac{a_2}{a_1}=\dfrac{12}{3}=4$ and $ n=7$
Now, $ a_{n}=3(4)^{n-1}$
Now, $ a_{7}=3 \times (4)^{7-1}=3 \times (4)^{6}=12288$
Our answers are: $ a_{n}=3(4)^{n-1}$ and $ a_{7}=12288$
