It is asked to sketch the root loci of the system . I have done that . Then it is asked to determine the stability of the system as function of $K$ .
After plotting I see that for $K>0$ it is unstable . But I cant determine a 'function' for stability . Is this solution ok .
When you're asked to determine the "stability as a function of $K$", you're not being asked for an explicit function. The question is equivalent to saying "state the dependence of the system's stability upon $K$". A solution (like you have) that gives a range of $K$ values for which the system is stable and a range for which it is unstable is a sufficient answer to this type of question.