Final answer:
The question aimed at identifying a function that is positive and has a decreasing positive slope at x = 3. Neither of the provided functions, y = 13x or y = x², matches this description, as one has a constant slope and the other has an increasing slope.
Step-by-step explanation:
The question asks to determine which explicit function describes a sequence where f(x) is positive with a positive slope that is decreasing in magnitude with increasing x. Analyzing the provided options, none of them directly corresponds to the described behavior of the function at x = 3. However, in general, for a function with a positive slope that decreases as x increases, we expect a function that has a decrease in its derivative as x increases, indicative of concave down behavior. Therefore, based on the common understanding of function behavior and slopes, neither option a) y = 13x with a constant slope nor option b) y = x² with an increasing slope would fit the requirement for a function that has a positive value at x = 3 and a positive but decreasing slope.