Never thought of drawing the lines to have clearer picture . Thank you so much

And in fact, it can show that area of A+B = ½×Length × breath.

Let the height for triangle B = u
Then height for A= Breadth -u
So area A = ½ × Length × (Breadth -u)
Area B = ½ × Length × (u)
Therefore, A+B
= ½ × Length × (Breadth -u) + ½ × Length × (Breadth -u)
= ½ × Length × (Breadth -u +u)
= ½ × Length × (Breadth)
= ½ × (rectangle's area)

Since A+B is half of rectangle, then C+D is another half. Which is another 56.

But a bit difficult to explain to primary student,