jmc

Follow PE(MD)(AS), doing the innermost parentheses first. $[(1\otimes 2)\otimes 3]-[1\otimes (2\otimes 3)]$ $[\frac{1^2}{2}\otimes 3]-[1\otimes \frac{2^2}{3}]$ $[\frac{1}{2}\otimes 3]-[1\otimes \frac{4}{3}]$ $[\frac{(\frac{1}{2}{)}^2}{3}]-[\frac{1^2}{(\frac{4}{3})}]$ $[\frac{1}{4}\cdot \frac{1}{3}]-[\frac{3}{4}]$ $\frac{1}{12}-\frac{3}{4}$ $\frac{1}{12}-\frac{9}{12}$ $\frac{-8}{12}$ $-\frac{2}{3}$, which is answer $\boxed{-\frac{2}{3}}$