Question 831856
"area of a pool is 4,000 linear feet" ---- area is SQUARE feet; NOT LINEAR feet.


Depth of pool being 12 linear feet means the distance from top to bottom is 12 feet.  


"if the depth is 12 linear feet and the SQUARE FEET OF THE POOL WALLS if the lining is 1"  ----- What do you mean?  What exactly is this?


Best thing to do is give the problem description and question exactly as it is written.  A figure or picture may also be needed.