Answer:
2π/7
Explanation:
The reference angle is the smallest angle between the terminal ray and the x-axis. It is always a positive value. Here, the terminal ray is in the 2nd quadrant, 2π/7 radians clockwise from the -x axis.
It makes an angle of 2π/7 radians with the -x axis, and an angle of 5π/7 radians with the +x axis. The reference angle is the smallest of these, so is 2π/7 radians.
The reference angle of -9π/7 is 2π/7 radians.