Greg is at his house. He gets in his car and drives north 9.7 miles and stops at the store. Then he turns around and drives south 7.6 miles to go to his friend's house.
How far is the friend's house from Greg's house?
North and South are complete opposites, so subtracting how far he went one way to the other will give you how far he would have originally driven if he had just gone straight to his friends home.
