Step-by-step explanation:
If triangles ADB and EDC are similar, then corresponding sides are proportional, from which we conclude:
BC/CD = AE/ED
BC / 21 = 6 / 18
Multiply by 21 on both sides,
BC = 6*21 / 18 = 21 / 3 = 7 units.
(note: Length BC is the same as length CB).
