Musical Acoustics, Combining frequencies of two instruments

[mhmil]

Homework Statement

A certain dog whistle operates at 21.5 kHz, while another (brand X) operates at an unknown frequency. If neither whistle can be heard by humans when played separately, but a shrill whine of frequency 5000 Hz occurs when they are played simultaneously, estimate the operating frequency of brand X. (Find your answer in kHz, not Hz, but do not enter units. So if you think the answer is 2000 Hz = 2.0 kHz, enter 2.0 )

Homework Equations

Unsure.

The Attempt at a Solution

I've tried 21.5khz + X khz = 5khz and solved for X, -16.5 khz. This doesn't make any sense, as how could it be negative? My homework doesn't accept this as an answer either (obviously.)

Any help would be greatly appreciated.

 

[rl.bhat]

Beats are the difference of two frequencies.
Fw2 = Fw1 + or - F(beat).

 

[mhmil]

Thanks!