Final answer:
The question asks for a linear model for shipping costs based on the number of books. Without specific data points, we cannot determine which of the given options is correct. To create a linear model, one needs the slope and y-intercept, which can be obtained from the cost at two different quantities of textbooks.
Step-by-step explanation:
The question provided is asking for a linear model that represents the shipping cost as a function of the number of textbooks ordered. Unfortunately, the needed data to determine the correct equation among the options (a, b, c, d) is not given in the question. To properly assist with this task, we would need the actual shipping costs associated with different quantities of books. However, we can discuss how to create a linear model in general.
A linear model can often be represented in the form y = mx + b, where m is the slope (change in shipping cost per book) and b is the y-intercept (base shipping cost). If we had the shipping costs for two different quantities of textbooks, we could calculate the slope (m) using the difference in cost divided by the difference in quantity. The y-intercept (b) could be found by taking one of the known costs and subtracting the product of the slope and the associated quantity.
Once we have m and b, we would plug these values into the linear equation to predict shipping costs for any quantity of books. It's also important to note that for homework problems, it's crucial to verify which equation fits the given data points most accurately, either by calculation or using graphical methods.