To find $ a_{ 4 } $ we use formula

$$ \color{blue}{a_n = a_1 \cdot r^{n-1}}$$

In this example we have $ a_1 = 2 ~~,~~ r = 3 ~~\text{and}~~ n = 4 $. After substituting these values to above formula, we obtain:

$$ \begin{aligned} a_n &= a_1 \cdot r^{n-1} \\ a_{ 4 } &= 2 \cdot 3^{ 4 - 1} \\ a_{ 4 } &= 2 \cdot 27 \\ a_{ 4 } &= 54 \end{aligned}$$

The first few terms of this sequence are:

$$ 2, ~~~6, ~~~18, ~~~54 . . . $$

Geometric sequences