Answer:
Question 11:
∠ABC = ∠EDC = 90° (given)
∠ACB = ∠ECD (vertically opposite angles)
∴ Triangle ABC is similar to triangle ECD. (AA Similarity Test)
Question 12:
Since triangles ABC and ECD are similar,
=
=

∴ DE = 9 ÷

= 15cm
∴ The length of DE is 15cm.