If $a^4 + a^2 b^2 + b^4 = 3$ and $a^2 + ab + b^2 = 3$, $a^2 - ab + b^2 = ?$

Note $a^4+a^2b^2+b^4=(a^2+ab+b^2)(a^2-ab+b^2)$. Substituting the given values, $3=3\times(a^2-ab+b^2)$, so $a^2-ab+b^2=1$.