The correct implementation is to implement the line amount as a numeric formula field and the order amount as a roll-up summary field. This is because formula fields can calculate values based on other fields, such as quantity and price, without requiring any code or process1. Roll-up summary fields can aggregate values from related records, such as the sum of line amounts for an order, without requiring any code or process2. This solution is the most declarative and efficient way to achieve the requirement.
References:
* 1: Create Formula Fields
* 2: Define Roll-Up Summary Fields