Simplify. (note: x ≠ 0) begin expression . . begin fraction . . 21 times x raised to the fourth power, plus 6 times x raised to the third power, minus 3 times x raised to the second power, plus 9 times x . . over . . 3 times x . . end fraction . . end expression a begin expression . . 7 times x raised to the third power, plus 2 times x raised to the second power, minus x, plus 3 . . end expression b begin expression . . 7 times x raised to the third power, plus 2 times x raised to the second power, plus x, plus 3 . . end expression c begin expression . . 18 times x raised to the third power, plus 3 times x raised to the second power, minus 6 times x, plus 6 . . end expression d begin expression . . 18 times x raised to the third power, plus 3 times x raised to the second power, plus 6 times x, plus 6 . . end expression