Exercise 9.5.1: Counting strings over {a, b, c}. About Count the number of strings of length 9 over the alphabet {a, b, c} subject to each of the following restrictions. (a) The first or the last character is a. (b) The string contains at least 8 consecutive a's. (c) The string contains at least 8 consecutive identical characters. (d) The first character is the same as the last character, or the last character is a, or the first character is a.