A = {{2,3,4},{1,-2,1}}

A//MatrixForm

(A = {{2,3,4},{1,-2,1}})//MatrixForm

B = {{1,-2,3},{4,5,0}}

B//MatrixForm

A+B

(A+B)//MatrixForm

2A-4B

%//MatrixForm

AA = (({{2, 4, 5}, {-1, 3, 0}, {1, 2, 4}}))

BB = (({{1, 0}, {0, 1}, {-1, 1}}))

BB

BB//MatrixForm

M1 = {{1,2,4},{3,4,5},{7,24,18}}

M1[[3]]

M1[[2,1]]

-7Times Row1 +Row 3-> Row 3

M2 = {M1[[1]],M1[[2]],-7M1[[1]]+M1[[3]]}

%//MatrixForm

M3 = {M2[[1]],-3M2[[1]]+M2[[2]],M2[[3]]}

%//MatrixForm

M4 = {M3[[1]],-1/2M3[[2]],M3[[3]]}

%//MatrixForm

M5 = {M4[[1]]-2M4[[2]],M4[[2]],M4[[3]]}

%//MatrixForm

M6 = {M5[[1]],M5[[2]],M5[[3]]-10M5[[2]]}

%//MatrixForm

M7 = {M6[[1]],M6[[2]],-1/45M6[[3]]}

%//MatrixForm

M8 = {M7[[1]]+3M7[[3]],M7[[2]]-7/2M7[[3]],M7[[3]]}

%//MatrixForm

M1//MatrixForm

RowReduce[M1]

%//MatrixForm

RowReduce[{M1[[1]],M1[[2]]}]

%//MatrixForm

RowReduce[{M1[[1]],M1[[3]]}]

%//MatrixForm

RowReduce[{M1[[2]],M1[[3]]}]

%//MatrixForm

M1//MatrixForm

M1[[All,3]]

%//MatrixForm

M1

M1[[All,{2,3}]]//MatrixForm

M1[[All,{1,3}]]//MatrixForm

M1[[{1,3},{1,3}]]//MatrixForm

M1//MatrixForm