I am trying to understand irreducibility of polynomials
and I am trying to solve the following question:

> 
> Decide if the polynomial $x^2-7$ is irreducible over $\mathbb Q, \mathbb R, \mathbb C$.
> 
> 
> 

For Q

I tried to use the Eisenstein’s Criterion,
7/7, but 7^2/7 is not, I believe it's irreducible over Q

For R is reducible
(x-sqrt{7})(x+sqrt{7})

For C

Not sure

Thanks in advance for your time.