The rules of a center-dot Sudoku are very similar to the rules of X-Sudoku puzzles and Hyper Sudoku puzzles. It is a standard Sudoku puzzle, but the center dot of each 3x3 region is filled in grey, and they make their own region, i.e. they additional grey cells must contain the numbers 1-9 once and only once.

This is an example of a finished center-dot Sudoku, and you can see that the grey center cells do contain the numbers 1-9 once and only once.

Have a look at the highlighted cell. We can use the normal rules of Sudoku to eliminate the numbers 1, 2, 3, 4, 5, 6 and 8. This leave us with just 7 and 9. However, there is a '9' in one of the other 'center-dot' grey square, so the highlighted cell must be a 7!

Have a look at this starting grid, can you work out the number that should go in the highlighted cell?