The IdentityMatrix(d) command returns a $dxd$ Identity Matrix. This command can be abbreviated as Id(d).

$\mathrm{with}\left(\mathrm{Student}\left[\mathrm{LinearAlgebra}\right]\right)\:$

$\mathrm{IdentityMatrix}\left(4\right)$

$\left[\begin{array}{cccc}1& 0& 0& 0\\ 0& 1& 0& 0\\ 0& 0& 1& 0\\ 0& 0& 0& 1\end{array}\right]$

$⟨⟨1\,2⟩|⟨3\,4⟩⟩-t\mathrm{Id}\left(2\right)$

$\left[\begin{array}{cc}1-t& 3\\ 2& 4-t\end{array}\right]$