Nopes. You got it wrong.
If a decision problem (yes/no) has both yes and no answers possible, it is a nontrivial problem. It can be either decidable, semi decidable or not even semi-decidable. The last 2 comes under undecidable category.
Now, if we can make a TM which can CORRECTLY detect both yes and no cases, then the problem is decidable. If the TM detects only the YES cases it is semi-decidable. If not all yes cases can be detected, then the problem is not even semi-decidable.
So to answer a question like this as decidable, you should give a TM or equivalently an algorithm for "how to decide".