A single equation with two variables, hence it can only be graphed and then interpreted from the graph.  The graph defines "legal values" for 'x' and 'y'.  If you zoom out further than the scope of the entire graph is easier to identify.  The vertex is at (4,-4) since any value for 'x' which is greater or less than '4' will return a value for 'y' which will trend towards positive 'y' values since the absolute value ensures that -4 is the lowest negative value possible for 'y'.