3.3 Lösning 2b

$\begin{array}{rclcl} 15 \, - \, x & = & 12 & \qquad | & + \, x \\ 15 \, - \, x \, + \, x & = & 12 \, + \, x & & \\ 15 & = & 12 \, + \, x & \qquad | & - \, 12 \\ 15 \, - \, 12 & = & 12 \, + \, x\, - \, 12 & \qquad | & \\ 3 & = & x \\ x & = & 3 \end{array}$