A={{2,3,1},{4,5,1},{8,-7,-2}}

Solve[{3x+4y==4,4x+3y==8},{x,y}]

Solve[{3x-5y+6z+2w==1,4x+3y-2z-w==2},{x,y,z,w}]

Solve[{3x-5y+6z+2w==1,4x+3y-2z-w==2},{x,y,z}]

Solve[{3x-5y+6z+2w==1,4x+3y-2z-w==2},{x,y}]

Solve[{3x-5y+6z+2w==1,4x+3y-2z-w==2},{x,w}]

ContourPlot[{3x+4y==4,4x+3y==8},{x,-5,5},{y,-5,5}]

ContourPlot3D[{3x-5y+6z==1,4x+3y-2z==2,x-y+2z==-1},{x,-5,5},{y,-5,5},{z,-5,5},Mesh->None]

Plot1 = ParametricPlot3D[{2t+1,3t-1,t+4},{t,0,1}]

(a*x+b*y+c*z==d)/.{x->2t+1,y->3t-1,z->t+4}

Expand[%]

Collect[%,t]

Plot2 = ContourPlot3D[{x+y-5z==-20},{x,-10,10},{y,-10,10},{z,-10,10},Mesh->None]

Show[{Plot1,Plot2}]

Plot3 = ContourPlot3D[{x+y-5z==-19},{x,-10,10},{y,-10,10},{z,-10,10},Mesh->None,ContourStyle->Green]

Show[{Plot1,Plot2,Plot3}]

Plot4 = ContourPlot3D[{2x+0*y-4z==-14},{x,-10,10},{y,-10,10},{z,-10,10},Mesh->None,ContourStyle->Magenta]

Show[{Plot1,Plot2,Plot3,Plot4}]

A={{2,3,1},{4,5,1},{8,-7,-2}}

A//MatrixForm

(A={{2,3,1},{4,5,1},{8,-7,-2},{4,-1,1}})//MatrixForm

A[[3,2]]

A[[All,1]]

A[[3,All]]

(B={{1,0,1},{2,0,5},{-6,4,1},{1,-1,1}})//MatrixForm

A//MatrixForm

(A+B)//MatrixForm

(B+A)//MatrixForm

(B1={{1,0,1},{2,0,5},{-6,4,1}})//MatrixForm

A+B1