So let's try out some different prioritization systems.
Left to right:
(((6 * 4) / 2) * 3) / 9
((24 / 2) * 3) / 9
(12 * 3) / 9
36 / 9 = 4
Right to left:
6 * (4 / (2 * (3 / 9)))
6 * (4 / (2 * 0.333...))
6 * (4 / 0.666...)
6 * 6 = 36
Multiplication first:
(6 * 4) / (2 * 3) / 9
24 / 6 / 9
Here the path divides again, we can do the left division or right division first.
Left first:
(24 / 6) / 9
4 / 9 = 0.444...
Right side first:
24 / (6 / 9)
24 / 0.666... = 36
And finally division first:
6 * (4 / 2) * (3 / 9)
6 * 2 * 0.333...
12 * 0.333.. = 4
It's ambiguous which one of these is correct. Hence the best method we have for "correct" is left to right.