HELP Two point source interference pattern

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Homework Statement

2 point sources 20cm apart produce 3 cm waves at a rate of 8 waves every 1.2 seconds

Homework Equations

how many nodal lines are present
what is the angular deviation of the 3rd nodal line?
what is the angle between the 3rd nodal line and the 5th nodal line?

The Attempt at a Solution

Answer for anyone who is wondering:
Nodal lines:
1 = (n-1/2) x wavelength
1 = (n-1/2) x 3
n = 7.17
Nodal Lines on both sides of right bisector, 7.17 x 2 = 14 (rounded down to nearest whole number)
Angular Deviation:
sinx = (n-1/2) x wavelength/d
x = sin-1(2.5 x (3/20))
x = 22 degrees
Angle between 3rd and 5th:
Repeat steps above to find angle of 5th nodal line and subtract angle of 4rd from angle of 5th

 

[haruspex]

It is not good enough just to post the question. you must post your attempt at a solution; or if you can't even get started explain why not. Is it that you don't understand some of the terms or concepts? E.g. do you know what a nodal line consists of?
To get you started, try http://www.physicsclassroom.com/Class/light/u12l1b.cfm