The equation you used is valid for speakers that are in phase. Here it states they are 180 degree out of phase which would mean d2-d1 = n*lambda Here d1 = 2.15m and d2 =sqrt(3.4^2+2.15^2) = 4m. So lambda = 4-2.15 = 1.85m So f = v/lambda = 343m/s/1.85m = 185.4Hz