please answer clearly and follow all directions

14. Find grammars for ={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. In each case, give convincing arguments that the grammar you give does indeed generate the indicated language