phi:Q[t] -> R be defined phi(t)=rt(2) and with elements of Q mapping themselves in R. Prove that rt(8) is in Im(phi)

let h=(t^4)+(t^4)-4 show phi(h)=0

Q is rationals, R is reals.

ok i can define Im(phi) and then i say rt(8)=phi*rt(2) => phi=2..im really lost help need help pretty quick, exam revision!