What is the domain and range and when is it decreasing?

The Domain is the left most part of the line to the right most part of the line (the most negative x, to the most positive x). 

The Range is the bottom most part of the line to the top most part of the line (the most negative y, to the most positive y).

Domain - [0,12]
Range - [-2,4]
It starts decreasing at (0,4) and stops decreasing at (3,-2)

Hope I helped!