1^2 + 4^2 + 7^2 + + (3n - 2)^2 = n(6n^2-3n-1)/2

Use math induction proof method will be usefully 

for n=1 will get 

1^2 = 1(6*1^2 -3*1 -1)/2
1 = 1(6-4)/2
1 = 1(2)/2
1 = 1

suppose true for n=k and prove for k = k+1 

hope this will help you