Cho hàm số \(\mathop {{\rm{lim}}}\limits_{x \to 2} \frac{{f\left( x \right) - 15}} = 3.{\rm{\;}}\) Tính \(\mathop {{\rm{lim}}}\limits_{x \to 2} \frac{{f\left( x \right) - 15}}{{\left( {{x^2} - 4} \right)\left[ {\sqrt {2f\left( x \right) + 6} + 4} \right]}}\). Đáp án: _______