Calculating Wavelength in a Ripple Tank: Understanding the PS1-PS2 Equation

[Quantum Fizzics]

Homework Statement

Two sources 6.0 cm apart, operating in phase, produce water waves. A student selects a point on the first nodal line and measures from it 30.0cm to a point midway between the sources and 5.0cm (on the perpendicular) to the right bisector.
a) what is the wavelength

Homework Equations

PS1 - PS2 = (n-1/2)lambda
lambda = xnd/L(n-1/2)

The Attempt at a Solution

I have tried to use both of these equations, & I still can't get the answer I don't understand.. the answer is 2.0cm and the answers I've been getting are 11.7cm, 60cm, 20cm... I don't understand

 

[Bystander]

Try giving us a few definitions: first nodal line; right bisector; perpendicular to the right bisector. Been a while since I played with a ripple tank, and I'm not remembering what some of the descriptive terms mean.

 

[haruspex]

A student selects a point on the first nodal line

Seems to me you need to be told it's the first point on the first antinodal line.

measures from it 30.0cm to a point midway between the sources and 5.0cm (on the perpendicular) to the right bisector

Maybe you've derived the wrong diagram. In Cartesian coordinates, if the sources are at (-3,0) and (+3,0), and we take the first node on the first nodal line in the first quadrant, where do you think the chosen node is?