a={{3,1,4},
{5,6,1},
{-1,2,-4}};
MatrixPower[a,-1]

b=Inverse[a]

c={{0,0,0},
{5,0,0},
{-1,2,0}};

(*d=Table[0,{3},{3}]*)
Clear[d];

d=c+Transpose[c]

{{0,5,-1},{5,0,2},{-1,2,0}}
MatrixForm[d]

θ=66/180;
T=N[a.RotationMatrix[θ,{1,0,0}],8]