Final answer:
To find the length of line segment DB, subtract the length of line segment DA from the length of line segment AB.
Step-by-step explanation:
The length of line segment DB can be calculated by subtracting the length of line segment DA from the length of line segment AB. Since the total distance covered by the truck is dAB + dBC, and the child is 50 meters ahead, the length of line segment AB is dAB - 50 meters. If we have the length of line segment DA, we can then calculate the length of line segment DB by subtracting DA from AB.