| $$ \begin{aligned}7x(x^2+9)& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}7x^3+63x\end{aligned} $$ | |
| ① | Multiply $ \color{blue}{7x} $ by $ \left( x^2+9\right) $ $$ \color{blue}{7x} \cdot \left( x^2+9\right) = 7x^3+63x $$ |