$b.\\\left[\begin{array}{ccc}1&3&11\\-2&8&-12\\3&11&40\\\end{array}\right] \xrightarrow{-(1)/(2)R_2}\left[\begin{array}{ccc}1&3&11\\1&-4&6\\3&11&40\\\end{array}\right] \\\\-(1)/(2)[-2\ 8\ -12]=[1\ -4\ 6]$

$c.\\\\\left[\begin{array}{ccc}1&3&11\\-2&8&-12\\3&11&40\\\end{array}\right]\xrightarrow{2R_1+R_2}\left[\begin{array}{ccc}1&3&11\\0&14&10\\3&11&40\\\end{array}\right] \\\\2R_1\to2[1\ 3\ 11]=[2\ 6\ 22]\\2R_1+R_2=[2\ 6\ 22]+[-2\ 8\ -12]=[0\ 14\ 10]$