If the roots of  x3−12x2+39x−28=0 are in A.P. , then their common difference, is

Let a - d, a, a+ d be the roots of the equation x3−12x2+39x−28=0. Thena−d+a+a+d=12 and, (a−d)a(a+d)=28⇒ 3a=12 and aa2−d2=28⇒ a=4 and d=±3