Answer
The solution set is: $\{5 \}$
Work Step by Step
Recall the rule that
$a^m=a^n \longrightarrow m=n$
if
$a\ne1,a\ne-1$
Applying this rule, we obtain:
$3^{x}=243 \\ 3^x=3^5$
The base is the same, so their exponents must be equal.
Therefore, $x=5$
So, the solution set is: $\{5 \}$
