In the What If? Section of Example 37.2, it was claimed that overlapping fringes in a two-slit

Question:

In the What If? Section of Example 37.2, it was claimed that overlapping fringes in a two-slit interference pattern for two different wavelengths obey the following relationship even for large values of the angle θ: A/ A = m`/m
(a) Prove this assertion.
(b) Using the data in Example 37.2, find the value of y on the screen at which the fringes from the two wavelengths first coincide.