Solve [{2 x - 2 y +  z -  w == 7,
      - x + 4 y - 5 z + 3 w == 1,
        x +  y -  z +  w == 4,
      -4 x + 2 y +  z - 2 w == 6}, {x, y, z, w}]
Solve[x^3-21x+20==0,x]
Plot[x^2-2x+3,{x,-2,2}]
Plot[Cos[2x],{x,-2,2}]
Plot[Sin [1/x],{x,-2,2},Axes->False]
Plot3D[x^2 + y^2, {x, -2, 2}, {y, -2, 2},Axes->False]
Plot3D[x^2 - y^2 + 2 x y ^2 + 1, {x, -2, 2}, {y, -2, 2}]