21. In a parallelogram, opposite angles are equal and adjacent angles are supplementary (add to 180°). m∠R = m∠P = 180° -m∠Q m∠Q = 107° m∠P = 73°
25. If you draw any diagonal, it divides the figure into two congruent triangles (by the SSS postulate). Corresponding angles are congruent, so the diagonal is a transversal between parallel lines. Thus opposite sides are parallel and the figure is a parallelogram.
27. Opposite sides are equal, and the diagonals bisect each other. Your four equations are 2w+4 = 5w-6 . . . . w = 10/3 12 = 3x-3 . . . . . . . x = 5 6y = 4y+6 . . . . . . . y = 3 27 = (5/4)z +2 . . . z = 20