Find the length of the missing side. hypotenus = 18m short leg = 11m

Assuming the triangle is right: Pythagoras:

sqrt (18^2-11^2) = sqrt (324-121) = sqrt (203). Does not look nice, like with integers, but that's how it is.

If it is not a right triangle, you would need some more info, one angle, for instance, so I think the above is correct