Answer:If the triangles are similar, the ratio of corresponding sides will be equal.
AC / DF = BC / EF
Substitute their values,
12 / 10 = 18 / EF
EF = 18/12 * 10
EF = 3/2 * 10
EF = 30/2
EF = 15 cm
In short, Your Answer would be 15 cm
Hope this helps!