In 1990, tests showed that water in a lake was polluted with 7 milligrams of mercury compounds per 1000 liters of water. Cleaning up the lake became an immediate priority, and environmentalists determined that the pollution level would drop at the rate of 0.75 milligrams of mercury compounds per 1000 liters of water per year if all of their recommendations were followed. If 1990 corresponds to x = 0, find the equation of the line that allows the environmentalists to predict the pollution level y in future years if their recommendations are followed. Sketch the graph of the equation and determine when the lake will be free of mercury pollution according to this graph.