Calculate the amount of formal languages Statement: 


  There are 256 formal languages $L$ with $L \subseteq \{w \in \{a,b\}^*
 \ \vert \ \ \vert w \vert = 3   \}$


Answer: 


  True.


My question is why there are 256 formal languages. If I calculate the possible combinations with length 3 I get 6 combinations:

$M =\{aaa, aab, abb, bbb, bba, baa\}$

Calculalting the power set results in: $2^M = 2^6 = 64$

Where did I go wrong the get $64$ as result?
That would be eight combinations, as $2^3=8$. You missed $aba$, and another one.