When representing waves in two dimensions, a ray can be drawn parallel to the crests

Let Karen's forward speed be considered as positive.
Therefore, before the headband is tossed backward, the speed of the headband is
V = 9 m/s

The headband is tossed backward relative to Karen at a speed of 20 m/s. Therefore the speed of the headband relative to Karen is
U = -20 m/s

The absolute speed of the headband, relative to a stationary observer is
V - U
= 9 + (-20)
= - 11 m/s

The stationary observes the headband traveling (in the opposite direction to Karen) at a speed of 11 m/s backward.