I have come across this question and actually dont think i understand completely what the question requires and how to approach such a question.Here is the question: let n∈N+
$$
H_n : { \begin{bmatrix}a&b\\c&d\end{bmatrix}} ∈ sl(n,Z)| a=d=1(modn), b=c=0(modn)
$$
is
$
H_n $ a subgroup of sl(2,Z).
(prove)
{sl(2,Z) : special linear group.} How do i approach such question and if someone can explain what the question is asking, paraphrase the question. What i understand now is that any identity matrix n by n is a subgroup of the the identity matrix of 2 by 2. Any help or simplification will be very much appreciated.