Factored version of y^2-2y-15

Answer: (y-5) (y+3)

/ / If the coefficient of the y^2 is 1 and equation has integer-solution, than 15 must be divisible to that solution. Fermat's Theorem consequence

It's obvious that y=5 is solution.

/ / If x1 and x2 are solution to quadratic equation, ay^2+bx+c, where a is 1, then x1*x2=c. Fermat's Theorem

Divide equation by (y-5), If you cannot do this look for other obvious solutions.