Prove that 3^(1/2) does not belong to the set Q[2^(1/2)]

Hi, I have this problem which I do not know how to solve... thanks for any help you can give me! And please, if you can solve it, I would really appreciate it if you could tell me the theorem you have used since I am feeling so lost in abstract algebra :(

"Let $\displaystyle Q[2^{1/2}]$ be the set consisting of all numbers of the form

$\displaystyle a+b(2)^{1/2}$

where a and b belong to $\displaystyle Q$ (the set of all rational numbers). Prove that $\displaystyle 3^{1/2}$ does not belong to $\displaystyle Q[2^{1/2}]$."