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