This division symbol in your expression causes the ambiguity in your expression. When using a division symbol, you must clarify your expression with the use of parentheses or brackets. The strict interpretation of your expression is the following:
\((y-a)^3-\frac{9}{3}(y-a)^2-6(y-a)-9\)
However, I could also imagine your expression any of the ones below:
\(\frac{(y-a)^3-9}{3(y-a)^2}-6(y-a)-9\)
\(\frac{(y-a)^3-9}{3(y-a)^2-6(y-a)}-9\)
\(\frac{(y-a)^3-9}{3(y-a)^2-6(y-a)-9}\)
Which one is it? Or is it something else?