Can you please help me understand how to answer this question please. Thank you. Plenty of thumbs up.

2. Consider a system where a password can only contain lower-case letters (a-z), upper-case letters (A-Z), digits (0-9), or the _ or - characters. (a) If a password must be exactly 8 characters long and each character is equally likely to occur in a password, what is the entropy of the system? (b) How many characters would be required so the system has at least 128 bits of entropy? (C) If a password is generated by combining four words where each word is equally likely to be chosen from a collection of 1024 possible words, what is the entropy of the system? (d) How many possible words would be required for this system to have at least 128 bits of entropy