Answer
$7x^2 + 3xlog_2x$ is $\Theta(x^2)$.
Work Step by Step
By the definition of $\Theta$: $f(x)$ is $\Theta(g(x))$ iff there exists a positive real number A, B, and a nonnegative real number k such that:
$A|g(x)| \leq |f(x)| \leq B|g(x)|$ for all real numbers $x > k$.
In this case $f(x) = 7x^2 + 3xlog_2x$, $g(x) = x^2$, A=1, B=10, and k=2.
