The expression is .

First add 

Both have different signs, so subtract the lesser absolute value from greater absolute value.

                          (Different signs: Subtract  from )

                                       (Since >, the sum has the same sign as )

The expression is changed to 

Both have different signs, so subtract the lesser absolute value from greater absolute value.

                        (Different signs: Subtract  from )

                                      (Since >, the sum has the same sign as 12) 

The sum is .