Explanation:
the relationship between x and y is defined by a constant factor.
you can "go" from x to y and from y to x consistently and without any interruptions by applying that constant factor.
that makes it a linear relationship (a line on a chart, hence the name).
proportional relationships are relationships between two variables where their ratios are equivalent. another way to think about them is that, in a proportional relationship, one variable is always a constant value times the other. That constant is know as the "constant of proportionality".
so, yes, it is proportional.
and the table is then
x y
0 0
3 3×2/3 = 2
6 6×2/3 = 4
9 9×2/3 = 6