Răspuns:
6cm.
Explicație pas cu pas:
[tex]\dfrac{AE}{EB}=\dfrac{AF}{FC},~=>~ \dfrac{AE}{AE+EB}=\dfrac{AF}{AF+FC},~=>~\dfrac{AE}{AB}=\dfrac{AF}{AC},~=>~ \dfrac{AE}{18} =\dfrac{7}{21},~=>~AE=\dfrac{7*18}{21}=6.[/tex]
Aplicate proporții derivate... :)) Succese!