$B = -\sqrt{x + 9} + 9$ $($ĐKXĐ: $x + 9 \ge 0$ hay $x \ge -9$$)$

Vì $\sqrt{x + 9} \ge 0 \quad \forall x \ge -9$

$\Rightarrow -\sqrt{x + 9} \le 0 \quad \forall x \ge -9$

$\Rightarrow -\sqrt{x + 9} + 9 \le 9 \quad \forall x \ge -9$

$\Rightarrow B \le 9$

Dấu "$=$" xảy ra khi và chỉ khi:

$x + 9 = 0$

$x = -9 \quad \text{(TMĐK)}$

Vậy $B_{\max} = 9$ khi $x = -9$

$\color{#FF0000}{♡^♡}\color{#FF7F00}{𝖘}\color{#FFD700}{𝖒}\color{#00FF00}{𝖕}\color{#00BFFF}{9}\color{#0000FF}{9}\color{#9400D3}{9}\color{#FF0000}{♡^♡}$