Double-slit question

[theonlytycrane]

I drew the same double-slit setup twice, but on the right I showed the angle as being between the axis and the line from the midpoint of the two slits and the 1st point of deconstructive interference.

I'm confused how the equation a * sin(theta) = n * lambda still holds as x != n * lambda and y != a in this case. Am I thinking about this correctly?

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