Lời giải:
$\frac{S_{AMN}}{S_{ANB}}=\frac{AM}{AB}=\frac{1}{2}$
Suy ra $S_{AMN}=\frac{1}{2}\times S_{ANB}$
$\frac{S_{ABN}}{S_{ABC}}=\frac{AN}{AC}=\frac{1}{3}$
$\Rightarrow S_{ABN}=\frac{1}{3}S_{ABC}$
Suy ra $S_{AMN}=\frac{1}{2}\times \frac{1}{3}\times S_{ABC}$
$\Rightarrow 6=\frac{1}{6}\times S_{ABC}$
$\Rightarrow S_{ABC}=36$ (cm2)