The expression $4x^3+21x-6$ has the same remainder when divided by $x-a$ or by $x+b$, where $a$ not equals to $b$. Find the value of $a^2+b^2-ab$. I have the answer but I don't know how to work through the question.
Given same remainder, by Polynomial Remainder Theorem we have,
$\displaystyle\ \ \ \ \ \ f(a)=f(-b)$