Answer
$a_n = 2^n$
Work Step by Step
First, find the pattern of the sequence in terms of how it relates to $n$, the order of the term in the sequence.
$a_1 = 2^1 = 2$
$a_2 = 2^2 = 4$
$a_3 = 2^3 = 8$
$a_4 = 2^4 = 16$
It looks like each term is $2$ raised to a power equal to $n$; therefore, the explicit formula is:
$a_n = 2^n$
