How to prove that a polynomial of degree n is θ(x^n)
Say $T(x) = a_n x^n + \dotsm + a_0$, then by the triangle inequality for $x \ge 1$:

$\begin{align}
   \lvert T(x) \rvert
     &amp;\le \lvert a_n \rvert x^n + \lvert a_{n - 1} \rvert x^{n - 1}
            + \dotsm + \lvert a_0 \rvert \\
     &amp;\le \lvert a_n \rvert x^n + \lvert a_{n - 1} \rvert x^n
            + \dotsm + \lvert a_0 \rvert x^n \\
     &amp;= (\lvert a_n \rvert + \lvert a_{n - 1} \rvert + \dotsm + \lvert a_0 \rvert)
           x^n
\end{align}$

I'm sure you can take it from here. You'll have to figure out an appropriate lower bound to match.