Final answer:
The error in the student's calculation was in the multiplication of negative numbers and in subtracting a negative number. The correct answer to the expression -3(-6)(-4)-(-9) is -72 + 9, which equals -63.
Step-by-step explanation:
The student's mistake in the problem -3(-6)(-4)-(-9) lies in the multiplication and subtraction of negative numbers. When two negative numbers multiply, the answer has a positive sign.
Here, -3 multiplied by -6 gives +18, and then multiplied by -4 gives -72. When subtracting a negative number, the subtraction sign changes to addition, as in subtracting -9 changes to adding +9.
Hence, the correct calculation should be:
- -3(-6)(-4) = 18(-4) = -72
- -72 - (-9) = -72 + 9 = -63
Therefore, the correct final answer to the expression is -63, not -81 as the student calculated.