If you draw lines from the ends of each point to the axis you can draw a square that surrounds the triangle. This gives you 3 right triangles, with the distance of each side.
Find area of each triangle using 1/2 bh. (3.5, 6, 6)
Then find the area of the square (28)
Subtract total area of triangles from total area of square (28- 3.5 - 6 - 6) = 12.5