Answer
The solution set is $\left\{5\right\}$.
Work Step by Step
Solve using the different properties of Algebra to find:
$5x=18-3
\\5x=15
\\x =\frac{15}{3}
\\x=5$
Check:
$5(3)+3\stackrel{?}{=}18
\\15+3\stackrel{?}{=}18
\\18 \stackrel{\checkmark}{=}18$