Marshall's mistake lies in his assessment that the growth rate of \( g(x) = 5x + 10 \) is always greater than that of \( f(x) = 6 \cdot 1.5^x \). The correct response to indicate his error is:
Marshall is only considering a small portion of the domains of the functions. If he went farther to the right on the graph, he would observe that the growth rate of \( f(x)=6 \cdot 1.5^x \) eventually surpasses the growth rate of \( g(x)=5x+10 \) for large values of x.
This option accurately describes the behavior of the two functions as \( x \) increases, where exponential growth (\( f(x) \)) will eventually outpace linear growth (\( g(x) \)).