How did we divide the remaining of the bottom base (10 above)
into 2 equal sides/bases of 3 for the two triangles I and III?
The video just says 10-4 = 6 and 6 divided by 2 = 3 for each of the 2 triangles.
It is an isosceles trapezium where triangle I and triangle III are congruent.
G is illustrating why the formula works that’s why he used that.
Another way to see it: Imagine flipping triangle I 180° (3-Dimensionally) about its base (using its base as a hinge) and joining it to triangle III by the hypotenuse. Basically, reorient trianlge I such that we can join it to triangle III by the hypotenuse. We form a rectangle with base 7 and height 5 which clearly gives area as 35.