$\text{a)}\text{P}={\text{a}}^2={\left(3\cdot {10}^3\right)}^2=3^2\cdot {10}^6=9\cdot {10}^6\left[{\text{cm}}^2\right]$

$\text{b)}\text{P}={\text{a}}^2={\left(5\cdot {10}^5\right)}^2=5^2\cdot {10}^{10}=25\cdot {10}^{10}=2,5\cdot {10}^{11}\left[{\text{mm}}^2\right]$

$\text{c)}\text{P}={\text{a}}^2-{\left(1,2\cdot {10}^6\right)}^2={\left(1,2\right)}^2\cdot {10}^{12}=1,44\cdot {10}^{12}\left[{\text{dm}}^2\right]$