I'm struggling writing grammars. I understand what its asking me but dont know how to write them.

14. Find grammars for E = {a, b} that generate the sets of (a) all strings with exactly two a's. (b) all strings with at least two a's. (c) all strings with no more than three a's. (d) all strings with at least three a's. (e) all strings that start with a and end with b. (f) all strings with an even number of b's