My answer is that k is greater than 6/7. I don't know, right? The process is as follows: there is 7 pie/12 between negative pie /3 and pie /4. Consider the extreme case: 7 pie/12 is exactly the half period of the function, and T/2=K/2=7 pie/12. The solution is K=7 pie /6. Then draw it and analyze it to know the value of k, sorry.