A useful method I called it the “rubber band” vertex rule, where the vertex of triangle moves horizontally along the parallel line (height) above the base and the base of triangle remains the same, area of triangle remains the same. (A key concept)
If we flip the bottom shaded triangle up, the height of both riangles are the same with the vertices along the top line. Joining both vertices on any point along that line will give a combined triangle in the TOP half the rectangle (Base of triangle is PQ).
Area is 1/2 x 15 x 3.5 = 26.25 sq cm#