\[\frac{x - 4}{\sqrt{x - 3} + 1} - \frac{x - 12}{3 + \sqrt{x - 3}}\]
\[y = \sqrt{x - 3};\ \ y^{2} = x - 3;\ \ x = y^{2} + 3:\]
\[\frac{y^{2} - 1}{y + 1} - \frac{y^{2} - 9}{3 + y} =\]
\[= \frac{(y - 1)(y + 1)}{y + 1} - \frac{(y - 3)(y + 3)}{y + 3} =\]
\[= y - 1 - y + 3 = 2.\]
\[Ответ:2.\]