asymptotic tight bound for quadratic functions

In CLRS (Introduction to Algorithms by Cormen, Leiserson, Rivest, and Stein), for a function

f(n) = an² + bn + c

they said

Suppose we take the constants c₁ = a/4, c₂ = 7a/4, and n₀ = 2·max(|b|/a, √(|c|/a)).
Then 0 ≤ c₁n² ≤ an² + bn + c ≤ c₂n² for all n ≥ n₀.
Therefore f(n) is Θ(n²).

But they didn't specify how values of these constants came ?
I tried to prove it but couldn't.
Please tell me how these constants came ?