(n - 1)! = (n -1)(n-2)(n- 3)(n-4)..........1
(n-3)! = (n-3)(n-4)(n-5)(n-6)........1
(n -1)(n-2)(n- 3)(n-4)..........1 = 5
(n-3)(n-4)(n-5)(n-6)........1
(n - 1)(n- 2) = 5
n2 - 3n + 2= 5
n2 - 3n + 2 - 5 = 0
n2 - 3n -3 = 0
????? n is supposed to be an integer. Are you sure you have the right problem?