Sinusoidal waves

[Nyasha]

The question and attempt to solution is on the attached image. I don't understand why the answer in the textbook is 1.5ym mine is 1.8 ym

 

[kuruman]

If φ₁ = 30^o and φ₂ = 80^o, then φ₂ - φ₁ is not 30^o as you claim in your solution.