solution;
The SI unit of measurement of length is meters and weight it is kilograms. So we have to convert mm to m and kilogram as it is.
[tex]0.631mm=0.631\times10^-3m\\
therefore,\\
\frac{0.631mm}{(8.60kg)^2}\\
\frac{0.631\times10^-3}{(8.60kg)2}\\
8.531638\times10^-6[/tex]
Now since we need 3 significant digits, 10-6 is not significant as exponent is not significant. So the required answer is 8.53.