What are witnesses C and k for an O-bound?
I am going to guess what $C$ and $k$ mean in your context:

$f(x) = O(g(x))$ if and only if $\exists C, k \;\; \forall x \ge k \;\; (f(x) \le C|g(x)|)$.

You are asked to find constants $C$ and $k$ and prove that for all $x \ge k$ you have $8x+9 \le C x^2$.

I am going to take $C = 1$ and $k = 9$. We have $x \ge 9$ so $C g(x) = C x^2 = x^2 \ge 9 x = 8x + x \ge 8x + 9 = f(x)$.