The highlighted cell is another cell that we can fill right away. We can look at the numbers already in that column, row and
3x3 region, and by a process of elimination, work out what the highlighted cell should be.
That column already contains 2, 3, 4, 6 and 7. That row already contains 5, 6 and 8. That 3x3 region already contains
6 and 9. We know the highlighted cell can't be any of these numbers, because we can only have one of each number in each
column, row, and 3x3 region. The highlighted cell can't be any of 2, 3, 4, 5, 6, 7, or 9. This leaves us with just the