$\sqrt{x - 3}$ - 2$\sqrt{x^2- 9}$ = 0 (đk: x > 3)
<=> $\sqrt{x - 3}$ = 2$\sqrt{x^2- 9}$
<=> x - 3 = 4($x^{2}$ - 9)
<=> x - 3 = 4(x - 3)(x + 3)
<=> x - 3 - 4(x - 3)(x + 3) = 0
<=> (x - 3)(1 - 4(x - 3)) = 0
<=> (x - 3)(4 - 4x) = 0
<=> \(\left[ \begin{array}{l}x -3=0\\4-4x=0\end{array} \right.\)
<=> \(\left[ \begin{array}{l}x=3\\x=1 (ktm)\end{array} \right.\)