On a statement in The Joy of Factoring By Samuel S. Wagstaff (Jr.)?

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In 'The Joy of Factoring By Samuel S. Wagstaff (Jr.)' on page $32$ it is mentioned that Euler, Legendre, Gauss, and Chebyshev have observed that in $$ax^2+bxy+cy^2=N$$ for two different $(x,y)$ pairs then we can factor $N$. What is the correct procedure for this?