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