Marshall is correct to note that he has only considered a limited portion of the domains of the functions. The function \( f(x) = 6 \cdot 1.5^x \) is an exponential function, while \( g(x) = 5x + 10 \) is a linear function.
Exponential functions grow significantly faster than linear functions as \( x \) becomes large. Therefore, while \( g(x) \) may appear to have a greater growth rate for small values of \( x \), \( f(x) \) will eventually surpass \( g(x) \) for sufficiently large values of \( x \).
Thus, Marshall's mistake lies in not considering the long-term behavior of the functions; if he analyzed the graph further to the right, he would observe that the growth rate of \( f(x) \) surpasses that of \( g(x) \).