Where did the 4*x^2 come from and How is the area 216

Question

I understand everything else but that.

Answer 1

I've answered your original question, referring to the first answer.
4x^2 is (2x)^2 and 2x is added to the original side of the garden because the path extends the length and breadth of the garden by the path width on each side.
The area of 216 applies to neither the original garden area nor the extended area.