Final answer:
To formulate an equation regarding Melanie's age, we need to know the context of the problem, as the equation would differ whether it's about her age in the past, probabilities, or any other scenario requiring mathematical analysis.
Step-by-step explanation:
If Melanie is 18 years old, an appropriate equation to represent a situation involving her age would depend on the context of the problem, which isn't provided in the question. For a simple example involving age, if we wanted to know how old Melanie was a certain number of years ago, we could use the equation 18 - x = Melanie's age x years ago. However, for problems involving more complex mathematical concepts such as probabilities, exponential functions, or quadratic equations, we would need additional information related to these contexts to form the correct equation.