Factorization of rings problem

[zetafunction]

Homework Statement

in order to factorize 20 on the rings of $$Q( \sqrt 2)$$ i must solve

Homework Equations

$$x^{2} -2y^{2}=10$$

The Attempt at a Solution

i do not know how to solve it, i have tried by brute force with calculator but can not get any response , the given hint is that $$x^{2} -2y^{2}=5$$ has no solution.

 

[matt grime]

I can only guess that you've factored 20 as 2*10, so why have you not factored 10 as 2*5?

 

[zetafunction]

according to the teacher, who put the exercise the diophantine equation

$$x^{2}-2y^{2}=5$$ has no solution on integers.