The language of the DFA M in Example 6.3.4 consists of all strings over $\{a, b\}$ with an even number of $a$ 's and an odd number of $b$ 's. Use Algorithm 7.2.2 to construct a regular expression for $L(M)$. Exercise 2.37 requested a nonalgorithmic construction of a regular expression for this language, which, as you now see, is a formidable task.