The possible value of the independent values is the domain of the graph.
In the given graph, the independent value is the time while the dependent value is the number of gallons of water in a pool.
Since the independent value is the time, the values run from 0 until 150 hours. In interval notation, this is [0, 150].
As mentioned above, the dependent value is the number of gallons of water in a pool. As we can see in the graph, the number of gallons starts from 0 gallons until 3000 gallons which is the maximum in the graph. In interval notation, this is [0, 3000].