Derivation for identities involving Stirling Numbers and Binomial Coefficients

Question

Looking for a combinatorial derivation of the identities:

$${n\brack l+m} {l+m\choose l} = \sum_k {k\brack l} {n-k\brack m} {n\choose k}$$

&

$${n\brace l+m} {l+m\choose l} = \sum_k {k\brace l} {n-k\brace m} {n\choose k}$$

Also if the original publication can be cited for the above-mentioned results, I found these on wikipedia page, which gives the reference to Concrete Mathematics (Graham, Knuth, Patashnik) but the book does not give further reference to the results.

Answer 1

These are Identities 205 and 204 in Proofs That Really Count: The Art of Combinatorial Proof. Their proofs are left as exercises, but hints are given in the back of the book.