S → aSbb → a(aSbb)bb → aa(ε)bbbb → aabbbb (wrong). So that’s 4 b’s, not 3.
: ( a^n b^m \mid n \ge 0, m \ge n, m \le 2n ) cfg solved examples
While working through these problems, students often make these mistakes: S → aSbb → a(aSbb)bb → aa(ε)bbbb → aabbbb (wrong)