Given two non-constant polynomials $P(x),Q(x)$ with real coefficients. For a real number $a$, we define $$P_a= \{z \in C : P(z) = a\}, Q_a =\{z \in C : Q(z) = a\}$$Denote by $K$ the set of real numbers $a$ such that $P_a = Q_a$. Suppose that the set $K$ contains at least two elements, prove that $P(x) = Q(x)$.