In order to read the vent tuning frequency, F(b), more precisely we can adjust the frequency display limits to zoom in on the frequency range containing the two impedance peaks as shown below:
At this screen we can determine that F(b), the tuning frequency of the vented box, is approximately 40 Hz by locating the impedance minimum between the two peaks and reading the frequency where the minimum occurs. The closely zoomed view makes it easier to read the frequency from the scale below.