| $$ \begin{aligned}-\sqrt{21}\cdot(-7+\sqrt{35})& \xlongequal{ }-(-7\sqrt{21}+7\sqrt{15}) \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}7\sqrt{21}-7\sqrt{15}\end{aligned} $$ | |
| ① | Remove the parenthesis by changing the sign of each term within them. |