Which result goes into the yellow box?

The following possibilities exist:

2 -  3  = -1
2 -  2  =  0
2 + 2  =  4
2 + 6  =  8

On first sight three solutions seem to be possible, however, only 8 is the right solution. Why?

+6 cannot be added either to 7, nor to 4, nor to 8, since each calculation would result in a double-digit value.  Since +6 can only be added to 2 the result has to be 8.