f(x) is the function that represents a given equation. It is defined more specifically by rules or conditions to satisfy the whole formula/equation.
In this case, f(x) has an inverse proportion with x (e.g. f(x) = 1/x). The second parameter says that f(x) = -15 when x is 30. Given that condition, just look for the formula, among the choices, that will result to -15 when x = 30. The answer is:
D. f(x) = -450/x
This is just -450/30 = -15
