The following function expresses an income tax that is 10% for incomes below $4000, and otherwise is $400 plus 30% of income in excess of $4000. Jo.10x if 0 < x < 4000 400+ 0.30(x - 4000) if x 4000 (a) Calculate the tax on an income of $2000. (b) Calculate the tax on an income of $4000. (c) Calculate the tax on an income of $6000. (d) Graph the function.