Explanation:
I don't see the options I can select, but it should be something like "sum".
it is also the "sum" of the areas of the rectangles separated by the dashed line (6×5 and 6×4). so, 6(9) = 6(5) + 6(4).
it is strangely written.
but 6(9) simply means 6×9 or 6×(9). one could also write (6)(9). and the same for 6(5) and 6(4).