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:

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:

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:
![4.5*10^2cm^2\cdot\frac{10\text{ mm}}{1\text{ cm}}\cdot\frac{10\text{ mm}}{1\text{ cm}}=4.5*10^4\operatorname{mm}]()
Therefore we conclude that 70 squares inches is
![4.5*10^4\operatorname{mm}]()