• Right to left:

    6 * (4 / (2 * (3 / 9)))

    Nope! 6 × 4 ÷ 2 × 3 ÷ 9 =4 right to left is 6 ÷ 9 x 3 ÷ 2 × 4 =4. You disobeyed the rule of Left Associativity, and your answer is wrong

    Multiplication first: (6 * 4) / (2 * 3) / 9

    Also nope. Multiplication first is 6 x 4 x 3 ÷ 2 ÷ 9 =4

    Left first: (24 / 6) / 9

    Still nope. 6 × 4 x 3 ÷ 2 ÷ 9 =4

    Right side first: 24 / (6 / 9)

    Still nope. 6 × 4 x 3 ÷ 9 ÷ 2 =4

    And finally division first: 6 * (4 / 2) * (3 / 9)

    And finally still nope. 6 ÷ 9 ÷ 2 x 4 x 3 =4

    Hint: note that I never once added any brackets. You did, hence your multiple wrong answers.

    It’s ambiguous which one of these is correct

    No it isn’t. Only 4 is correct, as I have just shown repeatedly.

    Hence the best method we have for “correct” is left to right

    It’s because students don’t make mistakes with signs if you don’t change the order. I just showed you can still get the correct answer with different orders, but you have to make sure you obey Left Associativity at every step.