When a quadratic equation has a repeated solution?

hello : 
the discriminat of each quadratic equation : ax²+bx+c=0 (a ≠ 0) is :
Δ = b² -4ac

if: Δ = 0 : : x1 =x2 = -b/2a       (has a repeated solution)

(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

the answer is d

step-by-step explanation: