Answer: He took 9 instead of -9 and -7 instead of only 7.
Step-by-step explanation:
We know that in integers , we use negative(-) sign to represent the "down" or loss and positive(+) sign to represent "up" or "gain" in any amount.
Since , we have given that , Sebastian was in a hotel lobby and took the elevator up 7 floors to his room.
This will represent by 7 not -7 in integers.
Then he took the elevator down 9 floors to the parking garage.
This will represent by -9 not only 9 in integers.
Then , the movement of Sebastian is given by : +7+(-9)
Therefore, the Sebastian's error is : He took 9 instead of -9 and -7 instead of only 7.