Sound Waves-Destructive Interference hw help please

[reality99]

Homework Statement

Two loudspeakers are placed a distance of 5m above and below one another and driven by the same source. A detector is positioned a perpendicular distance of 12m from the two speakers and a vertical height 3.2m from the lower speaker. This results in constructive interference at the first order maximum. What minimum distance (in any direction0 should the top speaker be moved in order to create destructive interference between the two speakers?
*Velocity of sound=345m/s

Homework Equations

This is what the final equation comes out to be, I just don't know how to derive it.

distance apart=dA
detector placement distance=dD
detector vertical height=dH

((SQRT(dD^2+((dH-dA/2)+dA/2)^2)-(SQRT(dD^2+((dH-dA/2)-dA/2)^2)/2)

The Attempt at a Solution

I honestly have no clue how to do this or even where to start. I know the final equation and the answer, just not how to get to it. Any help is appreciated. (equation is above)

 

[Redbelly98]

Welcome to PF.

I'd start with the conditions for constructive and destructive interference to happen.