Answer
$$3x(x-3)+(2x+6)(-x-3)=x^2-21x-18$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}3x(x-3)+(2x+6)(-x-3)& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}3x^2-9x-2x^2-6x-6x-18 \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}3x^2-9x-2x^2-12x-18 \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle3}{\textcircled {3}} } }}}x^2-21x-18\end{aligned} $$ | |
| ① | Multiply $ \color{blue}{3x} $ by $ \left( x-3\right) $ $$ \color{blue}{3x} \cdot \left( x-3\right) = 3x^2-9x $$ Multiply each term of $ \left( \color{blue}{2x+6}\right) $ by each term in $ \left( -x-3\right) $. $$ \left( \color{blue}{2x+6}\right) \cdot \left( -x-3\right) = -2x^2-6x-6x-18 $$ |
| ② | Combine like terms: $$ -2x^2 \color{blue}{-6x} \color{blue}{-6x} -18 = -2x^2 \color{blue}{-12x} -18 $$ |
| ③ | Combine like terms: $$ \color{blue}{3x^2} \color{red}{-9x} \color{blue}{-2x^2} \color{red}{-12x} -18 = \color{blue}{x^2} \color{red}{-21x} -18 $$ |
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