Solving 2D Wave Interference - Find Wavelength & Speed

[F.B]

I just learned interference of waves in two dimensions, and i don't really understand it. Well i know how to use the formula and everything but i don't know what I am given in the questions. So can you help me with my given.

1. Two point sources, 5.0 cm apart, are operating in phase, with a common frequency of 6.0 hz, in a ripple tank. A metres stick is placed above the water, parallel to the line joining the sources. The first nodal lines (the ones adjacent to the central axis) cross the metre stick at the 35.0 cm and 55.0 cm marks. Each of the line joining the two sources. Determine the wavelength and speed of the waves.

I know i have to use this formula:

X/L=(n-1/2)lamda/d

i know that:
n=1
f=6
d=5
But i don't know what the rest of the numbers are.

My second questions.

Two very small, identical speakers, each radiating sound uniformly in all directions, are placed at points S1 and S2. The speaks are 5 m apart and are connected to an audio source so that they radiate in phase, at a common wavelength at 2.00 m. Sound propogates in air at 338 m/s
a) Point M, a nodal point is 7.0 m from S1 and more than 7 from S2. Find 3 possible distances M could be from S2.

I know that i am given:
d=5m
MS1=7
MS2= x+7
lamda=2m

I don't know what n is so i don't know how to solve this question so can anyone please help me with both these questions.

 

[Astronuc]

Think of the sound waves as concentric rings spreading outward both each source. Allow the radii of the rings to correspond to integral numbers of wavelength (because they are in phase), and see where they intersect.

The node lines are where the peaks of the wave sets intersect.