Choosing Constant for Last Step in Substitution METHOD $T(n)= 5T(n/4) + n^2$
Your constant $c$ needs to satisfy
$$ \frac{5}{16} c n^2 + n^2 \leq c n^2, $$
that is,
$$ \frac{5}{16} c + 1 \leq c. $$
Hopefully you can determine the set of constants $c$ for which this inequality is satisfied.