P(x)-Q(x) is a polynomial of degree at most n. P(x)-Q(x)=0 must have at most n solutions. But we know that it has at least (n+1) solutions. This is possible iff P(x) and Q(x) are identical (in which case the equality will hold for any value of x and there will be infinitely many solutions).
Next time plz post the question.
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