Can somebody me. i have less then 10 mins to get the right answer

The stack is 3 inches high, and thus has four vertical rectangular areas 3 by 1 in^2 each, which constitutes a total vertical surface area of 4(3 in^2), or 12 in^2. Twice the area of the base is 2(1 in^2) = 2 in^2.

Combine 12 in^2 and 2 in^2:        14 in^2  is the total surface area of this 3-d object.

(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