area of the big triangle (shaded area and small tris for below) is 36 * 51 divided by 2 (area of triangle is base * height over 2). this would give you 918.
area of the small triangle below is 51 * 11 divided by 2 (see above), which is 280.5.
shaded shape area is the big triangle shape minus the small triangle below, which is 918-270.5, which is 637.5 units.
