It’s the colours that stop you spotting it. It actually kinks both ways. The two small triangles have different gradients. One is 2/5 and one is 3/8. The total area for the shapes is 32. One 8×3 triangle, area 12, two L shaped pieces, total area 15, one 5×2 triangle, area 5. The area of the supposed big triangle is 0.5x13x5=32.5
The half each way is down to whether the hypotoneuse is convex or concave.