documentclass{article}
usepackage{nicematrix}
NiceMatrixOptions{transparent,nullify-dots}

newcommand{underbracedmatrix}[2]{%
    left(;
    smash[b]{underbrace{
            begin{NiceMatrix}[columns-width = auto]#1end{NiceMatrix}
        }_{#2}}
    ;right)
    vphantom{underbrace{begin{NiceMatrix}#1end{NiceMatrix}}_{#2}}
}

begin{document}
begin{equation}
K=
underbracedmatrix{
    3  & 1 &      &      &   \
    1  & 2 &  1   &      &   \
    & 1 &ddots&ddots&   \
    &   &ddots&  3   & 1 \
    &   &      &  1   & 1 \}{n}
end{equation}
end{document}

documentclass{vertex4-1}
usepackage{nicematrix}
NiceMatrixOptions{transparent,nullify-dots}

newcommand{underbracedmatrix}[2]{%
    left(;
    smash[b]{underbrace{
            begin{NiceMatrix}[columns-width = auto]#1end{NiceMatrix}
        }_{#2}}
    ;right)
    vphantom{underbrace{begin{NiceMatrix}#1end{NiceMatrix}}_{#2}}
}

begin{document}

begin{equation}
K=
underbracedmatrix{
    3  & 1 &      &      &   \
    1  & 2 &  1   &      &   \
    & 1 &ddots&ddots&   \
    &   &ddots&  3   & 1 \
    &   &      &  1   & 1 \}{n}
end{equation}

end{document}