In the last row, why did they convert 1 to 2's complement and not the zero?

and how we decide which one to convert to 2's complement in these type of questions? (row 4&5)

Solution - In Class Activity 4-12: M A 0 0111 1000 swat 1 1100 1 0101 1 0000 Sign B Sum cu Output in Decimal + 0110 11010 7+6= 13 + 1001 0001 1 8+9= 16+1 - 1000 0100 1 12-8= 4 - 1010 1011 0 5-10= -5 (in 2's comp.) - 0001 1111 0 0-1= -1 (in 2's comp.)