For (ab* + b)*, the language it represents is (a, b, ab, ba, aa, ab, aab,.....) i.e. all strings consisting of a and b.
And for (a + b)*, the language it represents is also (a, b, ab, ba, aa, ab, ...) i.e. again all strings consisting of a and b. Since both the regular expressions represent the same language, they are equivalent.