3.3 Lösning 1a

\[\begin{array}{rclcl} x \, + \, (x \, + \, 6) & = & 12 & & \\ x \, + \, x \, + \, 6 & = & 12 & & \\ 2\,x \, + \, 6 & = & 12 & \qquad | & {\color{Red} {- \, 6}} \\ 2\,x \, + \, 14 \, {\color{Red} {- \, 6}} & = & 12 \, {\color{Red} {- \, 6}} & & \\ 2\,x \, & = & 6 & \qquad | & {\color{Red} {/ \; 2}} \\ \displaystyle \frac{2\,x}{{\color{Red} {2}}} & = & \displaystyle \frac{6}{{\color{Red} {2}}} & & \\ x \, & = & 3 & & \end{array}\]