∠BAC ~ ∠EDF means that the triangles are similar. So the legs of the triangle share the same proportions, even if the sizes are different.
Since they share the same proportions, the same operations can be performed on each base to find the area.
The answer can be found with the knowledge that the area of a triangle is half of the height * width. You know the width of ∠BAC is 4, and the area is 8, so 16 (the area doubled) / 4 is the height. The width and height of ∠BAC are the same, and since the proportions are also the same, the width and height of ∠EDF are both 3. So the area is the width (3) times the height (3) divided by 2.
