Q 8.1  3dof with additional elements   p.203

mm = 
        DiagonalMatrix[{10 + Δm, 20, 
            30}];
MatrixForm[mm]
kk = {{50 + k1, -10, -40}, {-10, 
            60, -50}, {-40, -50, 
            90 + k2}};
MatrixForm[kk]

b = {1, 0, 0};    

    c = {0, 0, 1} ;

gg = 
      Simplify[c . Inverse[mm s^2 + kk] . 
            b /. {Δm ->  2, k1 ->  10, 
            k2 ->  20}]