Oliver thinks back and knows that it took him 4 miles east from his house to get to get to the pizzeria and 2 miles west to get to Barry's house.
So Oliver adds the total distance using the equation: d1 + d2 = dt.
So he adds 4 miles + 2 miles = 6 miles because he walked 4 miles east from his house to the pizzeria and he adds the 2 miles west because he walked back to Barry's house. So once he adds everything, the total distance is 6 miles
Oliver knows that he walked 4 miles east and then 2 miles west back to Barry's house. So he's trying to find the displacement from his house to Barry's house.
Oliver uses the formula Δx = xf −x0 to find the displacement from his house to Barry's house.
He uses Δx = xf −x0. So he subtracts 4 miles, which is his distance from his house to the pizzeria by 2 miles which is from the pizzeria to Barry's house. So Δx = 4f - 20 = 2 miles. The displacement is 2 miles between his house and Barry's house.