#### Answer

identity;
the set of all real numbers

#### Work Step by Step

Using the properties of equality, the given equation, $
5+3(2x+6)=5+3(2x+6)
,$ is equivalent to
\begin{array}{l}\require{cancel}
5+6x+18=5+6x+18
\\\\
6x-6x=5+18-5-18
\\\\
0=0 \text{ (TRUE)}
.\end{array}
Since the solution above ended with a TRUE statement, then the given equation is an identity. The solution is $\text{
the set of all real numbers
}.$