shaded is a triangle
if it is not, then missing info of the Q

Based on the diagram, there is no assumption made regarding the length of the side of the middle square ("5m" clearly stated in the diagram) and the difference in height between the square to the right and the middle square (again, "8m" and "5m" are clearly stated in the diagram, so the difference is irrefutably 3m). Take note that the question specifically states that the squares are placed side-by-side.

There is no mistake in using those said number. Common sense should prevail. Regards.

Yes. Side by side stated in the question
Well
Similar triangle
2:5=3:8 ?
(Again this is based on the sense that the shaded is a triangle....)

Thanks for your notes.

Well, you assumed without support that the shaded shape is a triangle. I am fully aware that it may not be a triangle since the question does not state it, and hence calculated accordingly. Regards

Please reread the question carefully. It asks for the area of the "shaded figure", not shaded triangle. Regards