Quadrilateral rstu is a parallelogram. what must be the value of x?

In this question, the  given quadrilateral is a parallelogram. And in a parallelogram, diagonals bisects each other. Therefore,

the parts of the diagonal with lengths

have to be equal. That is

So out of the given options, correct option is the third option .

(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