My late contribution:

The $6$ supplied by other answers could have been solved without filling in any possibles. There are three numbers $3,6,7$ missing from the centre column, and the $6$ can only go in the top cell.

Similarly there are three numbers $3,7,9$ missing from the second column, and the $3$ can only go in the cell at the top of the bottom block (yet you marked the possibilities $3,7$).

So this is one of my techniques: for any line, column or block already well populated, see where the missing ones can fit.

My late contribution:

The $6$ supplied by other answers could have been solved without filling in any possibles. There are three numbers $3,6,7$ missing from the centre column, and the $6$ can only go in the top cell.

Similarly there are three numbers $3,7,9$ missing from the second column, and the $3$ can only go in the cell at the top of the bottom block (yet you marked the possibilities $3,7$).

So this is one of my techniques: for any row, column or block already well populated, see where the missing ones can fit.

Allied to this, is that when you have a duplicate pair possible in any row, column or block, as you have with the $3,7$ in the centre column, you can remove all other $3$s and $7$s in that element - here leaving that lone $6$.

The same goes in the rarer case when you spot a set of three numbers appearing as the only possibles in three cells.

My late contribution:

The $6$ supplied by other answers could have been solved without filling in any possibles. There are three numbers $3,6,7$ missing from the centre column, and the $6$ can only go in the top cell.

Similarly there are three numbers $3,7,9$ missing from the second column, and the $3$ can only go in the cell at the top of the bottom block (yet you marked the possibilities $3,7$).

My late contribution:

The $6$ supplied by other answers could have been solved without filling in any possibles. There are three numbers $3,6,7$ missing from the centre column, and the $6$ can only go in the top cell.

Similarly there are three numbers $3,7,9$ missing from the second column, and the $3$ can only go in the cell at the top of the bottom block (yet you marked the possibilities $3,7$).

So this is one of my techniques: for any line, column or block already well populated, see where the missing ones can fit.