Ta có:

a.  $\sqrt{54}=\sqrt{3^{2}.6}=3\sqrt{6}$

b.   $\sqrt{108}=\sqrt{6^{2}.3}=6\sqrt{3}$

c.  $0,1\sqrt{20000}=0,1\sqrt{100^{2}.2}=10\sqrt{2}$

d.  $-0,05\sqrt{28800}=-0,05\sqrt{120^{2}.2}=-6\sqrt{2}$

e.  $\sqrt{7.63.a^{2}}=\sqrt{7^{2}.3^{2}.a^{2}}=7.3.\left | a \right |=21\left | a \right |=\left\{\begin{matrix}21a(a\geq 0) & \\ -21a(a<0) & \end{matrix}\right.$