A co-terminal angle is basically any radian value that you can get by either adding or subtracting a full rotation from the original one.
In this case, you can do 2pi/7 + 2pi, or 2pi/7 - 2pi.
Your answer is either 16pi/7 or -12pi/7. There’s actually an infinite amount of answers you could get, but these two are the easiest to come by.