Which operation transforms (1236)\begin{pmatrix} 1 & 2 \\ 3 & 6 \end{pmatrix}(1326) into (1200)\begin{pmatrix} 1 & 2 \\ 0 & 0 \end{pmatrix}(1020)?
R2←R2−3R1R_2 \leftarrow R_2 - 3R_1R2←R2−3R1
R2←R2+3R1R_2 \leftarrow R_2 + 3R_1R2←R2+3R1
R2←3R1−R2R_2 \leftarrow 3R_1 - R_2R2←3R1−R2
R2←R1−3R2R_2 \leftarrow R_1 - 3R_2R2←R1−3R2