Daily Dose of Sudoku

there are all others numbers, except for 7, in the same line/column/ $3\times3$ section with green box, so 7 is the only choice for the green box.

For the green box, the only possible values are 1, 5, 6, 7 and 8(since they are not in that block)

Now if we look at the row containing the green box, it has 1, 8 and 6. So the only possible values are 7 and 5.

Now if we look at the column containing the green box, it has 5. So the only possible value is 7.

Hence the answer is $\boxed{7}$

One may be tempted to finish the whole sudoku (big congrats if you have), but you just need to realize that there is only one box that $7$ could fit in. The diagram below shows the boxes in which $7$ can’t be placed.

As you can see, there is only one possibility.