I need very fast help, like within the hour, please!

[GCDEF]

You've been told several times you don't need a loop that increments the column variable. You've also been told several times that to access the third column, the index should be 2, not 3.

This is what your summing loop should look like.

for(i=0; i<RIDU; i++)
{
    nTotal += maatriks[i][2];
}