How Does Destructive Interference Occur With Different Sound Wavelengths?

[keemosabi]

Homework Statement

Two speakers, one directly behind the other and along the x-axis, emit sound waves along the x-axis. The speaker in the back emits sound with a wavelength of .02 m and the speaker in the front emits waves with a wavelength of .05m. If the two speakers are separated by 3 m, where along the x-axis will destructive interference first occur?

Homework Equations

d sin(theta) = m(wavelength)

The Attempt at a Solution

How do I use the above equation if there are two wavelengths that I need to use?

 

[keemosabi]

Am I using the correct equation?