Question 319001
Your calculation is correct. 
Check the setup of the problem. 
If angle A is 90, and it's opposite to side a, then it's the hypotenuse (the longest side).
{{{a^2=b^2+c^2}}}
{{{312^2=216^2+c^2}}}
{{{c^2=50688}}}
{{{c=225.1}}} mm
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