How can I prove that a figure is trapezium?​

To prove that it is a trapezoid, remember that the definition of a trapezoid is a quadrilateral with exactly one pair of parallel sides. Therefore, to prove it is a trapezoid you must verify that one pair of sides is parallel. From algebra, remember that two lines are parallel if they have the same slope. And also check for these The quadrilateral has two parallel sides.
The lower base angles are congruent and the upper base angles are congruent.

The correct answer is -20y