Sebastian was in a hotel lobby and took the elevator up 7 floors to his room. Then he took the elevator down 9 floors to the parking garage. He described his movement with the expression below. 9 + (negative 7) What is Sebastian’s error? Sebastian should have used –9 and 7. Sebastian should have started at zero before adding –7. Sebastian should have switched the two addends and written Negative 7 + 9. Sebastian should have used 0 and –9.