It is given that DE || BC and DE : BC = 4 : 5
Let ∆ ADE and ∆ABC
∠ADE = ∠ABC [corresponding angles]
∠AED = ∠ACB [corresponding angles]
∴ ∆ ADE ~ ∆ABC [by AA similarity]
We know that the ratio of two similar triangles is equal to the square of the ratio of their corresponding sides.