Final answer:
The function that has a positive value and a positive slope that decreases as x increases, given the options, would be 'y = x²'. This is because a quadratic function's slope varies, as opposed to a linear function with a constant slope.
Step-by-step explanation:
The original question seems to be incomplete, so I will address the provided reference information that aligns with the knowledge of writing functions and analyzing their behavior:
At x = 3, if a function f(x) has a positive value and a positive slope that is decreasing in magnitude with increasing x, the option that could correspond to f(x) from the choices given would be b. y = x². This is because the function y = 13x would have a constant slope (since it's a linear function with a slope of 13), which doesn't fit the description of a decreasing slope. In contrast, the function y = x² has a positive slope that decreases in magnitude as x increases past the point (3, 9).