@DanielFischer I want to solve the recurrence relation $T(n)=7T\left( \frac{n}{3} \right)+n^2$.
I have tried the following:
$a=7 \geq 1, b=3>1, f(n)=n^2$
$n^{\log_b a}=n^{\log_3 7} \approx n^{1.77}$
So $f(n)=n^2 = \Omega(n^{\log_b a+ \epsilon})$, for $\epsilon=0.23>0$
$a f \left( \frac{n}{b}\right)=7 \left( \frac{n}{3}\right)^2=\frac{7}{9}n^2\leq cf(n)$ for any $c \in [\frac{7}{9},1)$.
So from the case 3 of Matser theorem, we have that $f(n)=\Theta(f(n))=\Theta(n^2)$.