Solve "cos 3x = -1" algebraically given the domain [0,2pi)

What is the value of x? enter your answer in the box. photo attached.

How do i do this activity, is appreciated

Jack has a piece of red ribbon that is two times as long as his piece of blue ribbon. he says that he can use two different equation to find out how long his piece of red ribbon is compared to his piece of blue ribbon is. is he correct? explain his reasoning