It is given that DB = 28 cm, AE = 20 cm and BC = 8 cm
AE and BC are perpendicular to diagonal BD
Area of quadrilateral ABCD = Area of Δ ABD + Area of Δ BDC
= 12 × DB × AE + 12 × DB × BC
= 12 × DB × (AE + BC)
= 12 × 28 × (20 + 8)
=14 × 28
= 392 cm2