Let x = width of walkway
The pool and the walkway combined form a square with sides of length 2x+6.
(2x+6)(2x+6) - (6)(6) = 253
4x2 + 24x + 36 - 36 = 253
4x2 + 24x - 253 = 0
x = [-24 ±√4624]/8 = [-24±68]/8 = 5.5 or -11.5
Since x can't be negative, x = 5.5 feet