| $$ \begin{aligned}3x^4(2x+5)& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}6x^5+15x^4\end{aligned} $$ | |
| ① | Multiply $ \color{blue}{3x^4} $ by $ \left( 2x+5\right) $ $$ \color{blue}{3x^4} \cdot \left( 2x+5\right) = 6x^5+15x^4 $$ |