Hello,

A matrix that is non square is not invertible.

That's the main difference.

As for your question.. :

You know that

Hence

By the associativity of matrix multiplication, this is :

Hence,a(*) solution is

(*) The reason why I say "a" and not "the", and your question states "a", is that it's not necessarily the only one. This is only one possible solution.

Does it look clear to you ?