4: Did you actually "cut off" a representative 9by12 rectangle (make it 3by4 inches!) to "see"?

Make EF = fold line (E on AB, F on CD) and make G intersection point of AC and EF.

Now go fold a 3by4 rectangle and see what you come up with...

5: HINT: Triangles ABC, ADE, BFG and CDG are similar right triangles; and AB = 5.

Let AE = x: then BF = 3 - x ; now have some fun...