12) E

We need to get rid of the 1 from the right hand side first.

So we get:

r – 1 =  (-) 6 Σd² 
            n (n² – 1)

We need to get rid of the (-) sign from RHS,

1 – r =  6 Σd² 
           n (n² – 1)

Solving n (n² – 1), we get = n³ – n
We need to get rid of (n³ – n) from the RHS.

(1 – r) (n³ – n) = 6 Σd²

We need to get rid of the 6 from RHS
Answer:

Σd² = (1 – r) (n³ – n)
                   6