How to get beat frequency from graph

[kthejohnster]

Homework Statement

how do you get the beat frequency from the beat envelope of two intersecting waves as in the attachment

Homework Equations

fb=f1-f2

The Attempt at a Solution

i thought maybe to get the frequency of the actual wave but don't think that's right

 

[Spinnor]

This gives a nice explanation, click on the link, "show",

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/beat.html#c1

Are you given any other information?