The line is completely horizontal. Every point on this line has a y coordinate of -3. The x value is allowed to be anything you want. In this case x = -2 and x = 3 is shown for the two points.
Since y is fixed to always being -3, this means y = -3 is the equation. If you want it to fit standard form then you can write it as 0x+1y = -3
Notice how 0x+1y = -3 matches up with Ax+By = C where
A = 0
B = 1
C = -3
Simplifying 0x+1y = -3 leads back to y = -3