Finding the last remaining number based on already confirmed numbers is very simple and it's the most basic technique used in Sudoku.
8's are already filled in as candidates in the G3 and F9 cells. This means that in the lower right hand box, an 8 is in the H7 cell.
In the B Line, there are no 5's. In the upper left and upper right boxes, there are already confirmed 5's, which leaves B5 and B6 as the only options. Since there is already a 5 in E5, the number 5 can't be in column 5, hence confirming that it is in B6.
Likewise, for E6, there are 2's in rows D and F and since there is a 2 in column 4, the 2 is therefore in E6, in the center block.
This method of eliminating potential numbers in cells for rows, columns and boxes, until the last possible number remains, is at the foundation of Sudoku and it will be used at all levels.
Most of the beginning level problems on this site can be solved, solely using this method.