I am assuming you mean hypotenuse of a right triangle with sides 12 and 9
using the pythagorean theorem
a^2 +b^2 =c^2 where a and b are the sides and c is the hypotenuse
12^2 +9^2 = c^2
144 +81 = c^2
225 = c^2
c=15
The hypotenuse is 15 mi
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