Square of the length of the greatest common tangent to the circles given by ${\mathrm{x}}^2+{\mathrm{y}}^2-4=0\text{ and }{\mathrm{x}}^2+{\mathrm{y}}^2-12\mathrm{x}+27=0$ is ____.

For circle,${\mathrm{k}}^2+{\mathrm{y}}^2-4=0$, centre is ${\mathrm{C}}_1(0,0)$and radius is ${\mathrm{r}}_1=2$.
For circle, ${\mathrm{x}}^2+{\mathrm{y}}^2-12\mathrm{x}+27=0$, centre is ${\mathrm{C}}_2(6,0)$and radius is ${\mathrm{r}}_2=3.$
Length of greatest common tangent 
$\begin{array}{l}=\sqrt{{\left({\mathrm{C}}_1{\mathrm{C}}_2\right)}^2-{\left({\mathrm{r}}_1-{\mathrm{r}}_2\right)}^2}\\ =\sqrt{36-1}=\sqrt{35}\end{array}$