15 ${\text{mm}}^{\text{2}}$  is equal to

${\text{15mm}}^{\text{2}}$

$\text{1m}=\text{1000mm}$

$\text{1mm}={\text{10}}^{-\text{3}}\text{m}$

$\text{15}\times {\left({\text{10}}^{-\text{3}}\text{m}\right)}^{\text{2}}=\text{15}\times {\text{10}}^{-\text{6}}{\text{m}}^{\text{2}}$

$\text{1m}=\text{100cm}$

$\text{1cm}={\text{10}}^{-\text{2}}\text{cm}$

$=\text{15}\times {\text{10}}^{-\text{6}}\times {\left({\text{10}}^{+\text{2}}\text{cm}\right)}^{\text{2}}$

$=\text{15}\times {\text{10}}^{-\text{6}}\times {\text{10}}^{\text{4}}{\text{cm}}^{\text{2}}$

$=\text{15}\times {\text{10}}^{-\text{2}}{\text{cm}}^{\text{2}}$

$\text{1km}=\text{1000m}$

$=\text{15}\times {\text{10}}^{-\text{6}}\times {\left({\text{10}}^{-\text{3}}\right)}^{\text{2}}{\text{km}}^{\text{2}}$

$=\text{15}\times {\text{10}}^{-\text{12}}{\text{km}}^{\text{2}}$