Answer:
f(l) = l/3
Explanation:
A clue to the functional relation is often found in the differences between values in the table.
Here the differences between "level" values are all 3. The differences between "triangles" values are all 1. The constant differences suggest the relation is linear. We note that all "triangles" values are 1/3 the "level" values.
f(l) = (1/3)l