Mary is making a decorative yard space with dimensions as shaded in green (ΔOAB).

Mary would like to cover the yard space with artificial turf (plastic grass-like rug).

Mary reasoned that she could draw a rectangle around the figure so that the point O was at a vertex of the rectangle and that points A and B were on sides of the rectangle. Then she reasoned that the three smaller triangles resulting could be subtracted from the area of the rectangle. Mary determined that she would need 28 square meters of artificial turf to cover the green shaded yard space pictured exactly.

Is Mary′s reasoning and claim correct? What is the correct evidence?