| $$ \begin{aligned}9uw^2x^3-21u^6w^6& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}-21u^6w^6+9uw^2x^3\end{aligned} $$ | |
| ① | Combine like terms: $$ -21u^6w^6+9uw^2x^3 = -21u^6w^6+9uw^2x^3 $$ |