To convert squared inches to squared centimeters we need to remember that one inch is equal to 2.54 cm.
Now that we know we can do the conversion:
[tex]70in^2\cdot\frac{2.54\text{ cm}}{1\text{ in}}\cdot\frac{2.54\text{ cm}}{1\text{ in}}=\frac{451.612in^2\cdot cm^2}{in^2}=451.612cm^2[/tex]Therefore 70 squared inches is equal to 451.612 squared cm (this is the exact result). If we use two significant figures then this will be equal to:
[tex]4.5\times10^2cm^2[/tex]Now, if we want to convert this to squared mm we need to remember that one cm is equal to 10 mm then we would have:
[tex]4.5\times10^2cm^2\cdot\frac{10\text{ mm}}{1\text{ cm}}\cdot\frac{10\text{ mm}}{1\text{ cm}}=4.5\times10^4\operatorname{mm}[/tex]Therefore we conclude that 70 squares inches is
[tex]4.5\times10^4\operatorname{mm}[/tex]