a)1. Run the TM on the string
w. If it ever enters its accept state, accept.
2. If it ever enters its reject, reject.
3) it can also go in infinite loop
Thus, we've proven that it is undecidable.
b)
Decidability is possible if the property is there for any recursively enumerable languages or if it is absent for all recursively enumerable languages (trivial)
since a given TM has a language which is REL
So TM accepts a REL is trivial property so it is decidable