Given the equation
$$\sqrt{x^{2} - 16} = 8 - x$$
$$\sqrt{x^{2} - 16} = 8 - x$$
We raise the equation sides to 2-th degree
$$x^{2} - 16 = \left(8 - x\right)^{2}$$
$$x^{2} - 16 = x^{2} - 16 x + 64$$
Transfer the right side of the equation left part with negative sign
$$16 x - 80 = 0$$
Move free summands (without x)
from left part to right part, we given:
$$16 x = 80$$
Divide both parts of the equation by 16
x = 80 / (16)
We get the answer: x = 5
Because
$$\sqrt{x^{2} - 16} = 8 - x$$
and
$$\sqrt{x^{2} - 16} \geq 0$$
then
$$8 - x \geq 0$$
or
$$x \leq 8$$
$$-\infty < x$$
The final answer:
$$x_{1} = 5$$