Ta có : 

a.   $\sqrt{16x}=8$

<=> $16x=8^{2}$

<=> $x=8^{2}:16$

<=> $x=4$

Vậy $x=4$ .

b.  $\sqrt{4x}=\sqrt{5}$

<=> $4x=5(5\geq 0)$

<=> $x=\frac{5}{4}$

Vậy $x=\frac{5}{4}$ .

c.  $\sqrt{9(x-1)}=21$

<=> $9(x-1)=21^{2}$

<=> $x-1=21^{2}:9$

<=> $x-1=49$

<=> $x=50$

Vậy $x=50$ .

d.  $\sqrt{4(x-1)^{2}}-6=0$

<=> $\sqrt{4(x-1)^{2}}=6$

<=> $2\left | 1-x \right |=6$

<=> $\left | 1-x \right |=3$

<=> $\left\{\begin{matrix}1-x=3 & \\ 1-x=-3 & \end{matrix}\right.$

<=> $\left\{\begin{matrix}x=-2 & \\ x=4 & \end{matrix}\right.$

Vậy $\left\{\begin{matrix}x=-2 & \\ x=4 & \end{matrix}\right.$