= Solution
For the <double-slit experiment> with oblique incidence, the <optical path difference> has an unknown incident contribution plus $a\sin\theta$ in the outgoing direction. It cancels between adjacent angular features. From a minimum to the next maximum, the path difference changes by $\lambda/2$:
$$
\boxed{\lambda=2a(\sin43^\circ-\sin17^\circ)
=4.68\times10^{-7}\,\mathrm m\simeq468\,\mathrm{nm}.}
$$
This <incidence-independent adjacent-fringe wavelength> uses the statement that intensity rises directly from the stated zero to the following peak. No absolute interference order or incidence angle is needed; nonadjacent extrema would require the number of intervening fringes.
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