Final answer:
Using the direct proportionality equation f = 6g, by substituting g with the value of 3, we find that f equals 18.
Step-by-step explanation:
Given that direct proportionality exists between two quantities f and g, we can use the given equation to find the value of f when g = 3. The equation provided is f = 6g.
To find the value of f, we simply substitute the value of 3 in place of g:
f = 6 × g
f = 6 × 3
f = 18
Therefore, when g equals 3, the value of f is 18.