By conservation of energy: 1/λ₁ = 1/λ₂ + 1/λ₃. 1/λ₃ = 1/300 − 1/496 = (496−300)/(300×496) = 196/148800. λ₃ = 148800/196 ≈ 759 nm ≈ 757 nm
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