Fit $y=\theta_0+\theta_1 x+\theta_2 x^2$ to $(1,-1),(0,1),(-1,1)$.
From $x=0 \Rightarrow y=1: \theta_0=1$.
From $x=1:-1=1+\theta_1+\theta_2 \Rightarrow \theta_1+\theta_2=-2$.
From $x=-1: 1=1-\theta_1+\theta_2 \Rightarrow-\theta_1+\theta_2=0 \Rightarrow \theta_2=\theta_1$.
Thus $2 \theta_1=-2 \Rightarrow \theta_1=-1$ and $\theta_2=-1$.
$$
y=1-x-x^2
$$
