Area of a trapezium = 1/2 × (sum of parallel sides) × height = 1/2 × (8+18) × 17 = 221 cm². Choose the correct formula first, substitute the given values carefully, and match the final unit with the type of measurement. The two parallel sides are averaged by adding them and multiplying by 1/2, then multiplied by the perpendicular height. Be sure to use only the parallel sides in the bracket and write the answer in square units.