Answer:
Both functions must have a constant rate of change.
The rate of change of either f(x) or g(x) must be 0.
Explanation:
We have that

and

From the first equation we know that both f (x) and g (x) must be of order 1 or of order 0, since the maximum exponent of X is 1. This indicates that both functions must have a constant rate of change , since the derivative of a function of order 0 or of order 1 is a constant.
From equation 2 we know that both f (x) and g (x) are composed of a single term, since the multiplication of both results in a single term (-9x). In addition, we verify that one of the functions is of order 0 and the other is of order 1, since the result of the product is of order 1.
If f (x) or g (x) are of order 0, which means that they are composed of only one constant, the rate of change of any of the 2 functions is 0.