Combinatorial Proof Based on the Binomial Formula

Please typeset your solutions with a math symbol editor (for example,
Latex)

For any answer you give, describe your solution process in words. NO
SOLUTION TO EVEN A SIMPLE PROBLEM SHOULD JUST HAVE ALGEBRAIC
SYMBOLS…EVERY SOLUTION NEEDS TO BE DESCRIBED IN WORDS.

For proofs, write out the proof as you normally would. Then, include an
explanation of your proof PRIMARILY IN WORDS. So you should have one
regular proof (the kind you see in textbooks), then another proof that
describes the proof mainly in words(you don’t see this that often).