>>8
真ん中の数字をnとすると

(n-1) + n + (n+1) + 470 = 3n +470
(n-1)^2 +n^2 +(n+1)^2 = 3n^2 +2

3n +470 = 3n^2 +2

3n^2 -3n-468=0
n^2 -n -156 =0
(n+12)(n-13) =0
n=13

12+13+14=39