0 3 3
1 7 6
2 11 9
3 15 12
4 19 15

y=3(i+1)+i
y=3i+3+i
y=4i+3

4i=y-3
i=(y-3)/4

0 5 5
1 9 8
2 13 11
3 17 14
4 21 17

3(i+1)+(i+2)
y=3(i+1)+i+2
y=3i+3+i+2
y=4i+5
i=(y-5)/4

0 
1
2
3
4