How to prove that language is decidable?
If you want to prove that a language is decidable, it is sufficient to construct a Turing Machine that accepts if the input is in the language. It MUST reject if the input is not in the language. 

Additionally, you are required to argue/prove that the Turing Machine constructed does that. Here's a hint: M must just halt on x, for it to be in L. Consider all possibilities of halting.