(n^3 - n)
= n(n-1)(n+1)
since it is divisible by 3 and 2 ,because it is a multiple 3 consecutive number
if, n-1 is odd then n+1 is also odd, then it is a multiple of 9
but if, n-1 is even then n+1 is also even, then it is a multiple of 4
so, largest multiple = max(9*2, 4*3) = max(18, 12) = 18