This is because this is written in slope intercept, which is defined as:
y = mx + b
With the slope being m and the y intercept being b. 1/3 is being added to -2/9x, and therefore by the rules of the formula it's the y intercept.
As a quick background on a y intercept, it's where the line crosses the y axis. It can be positive, or negative, and the coordinates never have an x value.
