In Introduction to Algorithms by CLRS, it's said
For any quadratic function $f(n)=an^2+bn+c$, where $a$, $b$ and $c$ are constants and $a>0$, $f(n)=\Theta (n^2).$ Formally, to show the same thing, we take constants $c_1=a/4, c_2=7a/4$ and $n_0 = 2 \cdot max(|b|/a, \sqrt{|c|/a}).$ You may verify that $0\leq c_1n^2\leq an^2+bn+c \leq c_2n^2$ for all $n\geq n_0.$
They didn't specify how values of these constants came? I tried to prove it but couldn't.