Let's look at the progression of f(x) values as the x value goes up. The x value goes up in steps of 1, but the f(x) value goes up in steps of 4. This means that the f(x) value grows 4 times faster than the x value. Comparing them side by side, you can see that each f(x) value is indeed 4 times larger than the corresponding x value. Therefore, the descriptive function is f(x)=4x.
