[tex]\begin{gathered} \frac{A}{C}=\frac{A}{B}\times\frac{B}{C} \\ \text{Thus,} \\ A\colon C=A\colon B\times B\colon C \end{gathered}[/tex][tex]\begin{gathered} A\colon B=\frac{A}{B}=\frac{3}{7} \\ B\colon C=\frac{B}{C}=\frac{4}{9} \\ \text{Thus,} \\ A\colon C=\frac{A}{C}=\frac{A}{B}\times\frac{B}{C} \\ A\colon C=\frac{3}{7}\times\frac{4}{9} \\ A\colon C=\frac{12}{63}=\frac{4}{21}=4\colon21 \end{gathered}[/tex]
Hence, A:C is 4:21