Resposta:

x = 18

Explicação passo a passo:

x - 3 = 15

x = 15 + 3

x = 18

[tex]\Large\displaystyle\text{$\begin{gathered} \sf{ x-3=15} \end{gathered}$}[/tex]

[tex]\Large\displaystyle\text{$\begin{gathered} \sf{ x=15+3} \end{gathered}$}[/tex]

[tex]\Large\displaystyle\text{$\begin{gathered} \boxed{\boxed{\sf{ x=18}}} \end{gathered}$}[/tex]