$\textit{Vì R1 nối tiếp R2 }$
$\textit{$\longrightarrow$ R12=R1+R2=6+3=9 $\Omega$ }$
$\textit{Vì R12 // R3}$
$\textit{$\longrightarrow$ Rtđ = $\frac{R12 \times R3}{R12 + R3}$}$
$\textit{$\longrightarrow$ Rtđ = $\frac{9 \times 5}{9 + 5}$}$
$\textit{$\longrightarrow$ Rtđ = $\frac{45}{14}$ $\Omega$}$