\[\boxed{\mathbf{51}\mathbf{.}}\]
\[1)\ \sqrt[3]{(x - 2)^{3}}\]
\[\textbf{а)}\ \ x \geq 2:\]
\[\sqrt[3]{(x - 2)^{3}} = x - 2.\]
\[\textbf{б)}\ x < 2:\]
\[\sqrt[3]{(x - 2)^{3}} = x - 2.\]
\[2)\ \sqrt{(3 - x)^{6}} = \sqrt{(3 - x)^{2 \bullet 3}} =\]
\[= |3 - x|^{3}\]
\[\textbf{а)}\ x \leq 3:\]
\[|3 - x|^{3} = (3 - x)^{3}.\]
\[\textbf{б)}\ x > 3:\]
\[|3 - x|^{3} = - (3 - x)^{3} = (x - 3)^{3}.\]
\[3)\ \sqrt[4]{(x + 6)^{4}} + \sqrt{(x - 3)^{2}} =\]
\[= |x + 6| + |x - 3|\]
\[\ - 1 < x < 2:\]
\[|x + 6| + |x - 3| =\]
\[= (x + 6) - (x - 3) =\]
\[= x + 6 - x + 3 = 9.\]
\[4)\ \sqrt[6]{(2x + 1)^{6}} - \sqrt[4]{(4 + x)^{4}} =\]
\[= |2x + 1| - |4 + x|\]
\[\ - 3 < x < - 1:\]
\[|2x + 1| - |4 + x| =\]
\[= - (2x + 1) - (4 + x) =\]
\[= - 2x - 1 - 4 - x = - 3x - 5.\]