It would probably help if the diagram you are supposed to be using is actually drawn to scale.
Even the corners which are cut away are not squares but rectangles.
Step 1 is straightforward enough.
Step 2: The height could be 4 units or approx 4.5 units depending on which dimension you use.
Step 3 is doable. Length of box is (30 – 2x) and width is (20 – 2x).