Are these sentences synecdoche or metonymy? "Lend me your ears" and "give me a hand." Please explain why.

Synecdoche is using the part for the whole, or vice versa, metonymy is the exchange of one word for another that is suggests.

"Lend me your ears" and "give me a hand" are both examples of synecdoche because they use the part of the person (ears and hands) to represent the whole person. They do not substitute "ears" or "hands" for other words, but instead figuratively link the body parts used for helping and hearing as substitutes for the whole person in question, if that makes sense.

This is a very subtle example, as it requires you to see the depth of the two phrases, but once you understand the reason why, it is quite simple.

I hope my explanation makes sense to you.