lowest frequency that produces an interference maximum frequency, produces, problem

amy

1. The problem statement, all variables and given/known data

A microphone is located on the line connecting two speakers that are 0.517 m apart and oscillating 180° out of phase. The microphone is 1.96 m from the midpoint of the two speakers. What is the lowest frequency that produces an interference maximum at the microphone's location?

What is the next lowest frequency that produces an interference maximum at the microphone's location? 2. Relevant equations

r2 -r1 = (n-0.5)(lambda)
lambda=v/f
v=343m/s

3. The attempt at a solution

r2 and r1 are the distances from the speaker
so r2 is .517/2 + 1.96 = 2.2185
and r1 is .517/2 + 2.2185 = 2.477

i know i have to solve for frequency so i plugged (343m/s)/f into the equation

but then i got stuck. what do i plug in for n and how i change n for the lowest frequency and the next lowest?  

    nevermind, figured it out

DISREGARD THE QUESTION!  

    Can you please tell me how to do this one? I've tried by using the equations but still can not figure it out.
Thanks!