1. Given the graph of a function, we have "visual" access to information about the function it represents. For example in this case, we can see that the function is decreasing, it only takes negative values (because it is entirely below the y axis) and we can also tell the domain and the range.
2. The domain is about the x- axis, and the range about the y axis.
3. In order to find the domain we project the graph into the x-axis, or we take the "shadow" of it into the x axis.
4. Doing so we see that the domain is x >= 0.
5. Remark: To find the range we would project the graph to the y axis, and get: y<=-1