Equivalent equations are equations that have the same solution sets. They are represented by the symbol "⇔". The document provides an example of equivalent equations 3x + 6 = 9 and x + 2 = 3, which have the same solution x = 3. It also discusses methods for solving equations with fractions, such as changing the equation into an equivalent equation without fractions by multiplying both sides by the lowest common multiple. Word problems can be solved by understanding the information given, representing the unknown as a variable, writing a mathematical model based on the information, solving the model, and checking the solution.