Try expanding $a(x+x_1)^2 + b(x+x_1)^2 +c$. Remember that $x_1$ is a solution of $ax^2 + bx + c = 0$, so $ax_1^2 + bx_1 + c = 0$. Try using this to simplify the expression obtained by expanding the second polynomial. You may want to use Vieta's formulas to find the final answer.