The given equation is y=3x.
This equation represents a direct proportion between variables, where x is time and y is speed. The walking rate is 3, which means he's waking at a rate of 3.
The point (3,9) represents that Alonso is walking 3 distance units in 9-time units. The point (5,15) represents that Alonso is walking 5 distance units in 15 distance units.