It is given to us that the Turing Machine halts for language L. That means that the language L is accepted by the turing machine. Any Language which is accepted by the turing machine is:decidable and recursively ennumerable i.e. Type 0 grammar.
But it is also given to us that this language is equal to L' which is undecidable language. Now a decidable language can be equal to an undecidable language in only one case when it is phi i.e. it contains nothing.So it is decidable as it is accepted by the turing machine and recursive. It is a type 0 grammar so it is not undecidable.
