If BC is parallel to DE then ∠D ≅ ∠B and ∠E ≅ ∠C. Therefore
ΔABC is similar to ΔADE.
The sides of smaller triangle are in proportion with sides of bigger triangle.
Therefore we have the equation:

We have AC = x, AE = x + 15, AB = 8, AD = 8 + 10 = 18.
Substitute:
cross multiply
use distributive property a(b + c) = ab + ac
subtract 8x from both sides
divide both sides by 10

Answer: AC = 12