need help with q15 and q21 please ?

For Exercises 15 thro 15. The set of strings over (a, b ugh 25, give a regular grammar that generates the described language. , c) in which all the a's precede the b's, which in turn precede the c's. It is possible that there are no a's, b's, or c's. 16. The set of strings over (a, b) that contain the substring aa and the subs 17. The set of strings over (a, b] in which the substring aa occurs at least twice. (Hint: tring blb. Beware of the substring aaa.) 18. The set of strings over (a, b) that contain the substring ab and the substring ba. 19. The set of strings over la, b) in which the number of a's is divisible by three. 20. The set of strings over (a, b) in which every a is either immediately preceded or immediately followed by b, for example, baab, a 21. The set of strings over (a, b) that do not contain the substring aba. 22. The set of strings over (a, b] in which the substring aa occurs exactly once. 23. The set of strings of odd length over (a, b] that contain exactly two b's. and b. 24. The set of strings over la, b, c) with an odd number of occurrences of the substring ab. 25. The set of strings over (a, b) with an even number of a's or an odd number of b's. 26. The grammar in Figure 3.1 generates (b'ab ab)+, the set of all strings with a positive even number of a's. Prove this. in Framnle 3.212 generates the prescribed language