Thank you for the clarification (and my apologies for the delay in my reply, but I only just now got back home).
If the length of the side of an interior (and square) panel is "y", then the area of one panel is y<sup>2</sup> and the area of all four panels is 4y<sup>2</sup>.
Looking at the graph, the width of the door is 4 + y + 4 + y + 4 = 2y + 12. So is the height. So what is the area?
Now subtract the panel area from the total area, set equal to the given "door less panels" area, and solve.
I don't know what they mean by "use substitution". Sorry.
Eliz.