Estimate the first three standing-wave frequencies of the vocal tract. use v=344m/s. (the answers are only an estimate, since the position of lips and tongue affects the motion of air in the vocal tract.)

Vocal tract behaves like a stopped pipe. 
The fundamental frequency for a stopped pipe is given with the following formula:

Higher harmonics are simply:

Keep in mind that the stopped pipe produces only odd harmonics. 
Lenght of a human vocal tract is 17cm(for males, females have slightly shorter vocal tract). We can now calculate the answers.

Now we can calculate higher harmonics: