216 sq. ft. in equal to how many linear feet?

Not fundamentally possible; length and area are totally different things.

The only way they are likely to both come in to the same calculation is something like material use or coverage, when working with something in strip or roll form - eg. like floor board planks or wallpaper etc.

That can be converted - IF you also know the width of the strip material, so you can convert a length of that to an area.

Without the width it remains impossible.

Square feet and linear feet have incompatible dimensions. There can be no conversion. Your question is comparable to asking how many hours are in a bushel.