Answer:
9
Explanation:
similar triangles.
that means there is one common factor to transform the lengths of all lines from one triangle to the other.
it looks to me DF is the longest side in DEF, and that would make it associated to BC of ABC.
so, the factor is then
BC = DF×f
24 = 12×f
=> f = 2
so, EF then is associated to AC.
AC = EF × f
18 = EF × 2
EF = 18/2 = 9