Step-by-step explanation:
If you draw an altitude of the right triangle, you get that the legs are (18/2)=9 and the height (which I'll call x) and a hypotenuse of 18.
By the Pythagorean theorem,
[tex] {9}^{2} + {x}^{2} = {18}^{2} \\ 81 + {x}^{2} = 324 \\ {x}^{2} = 243 \\ x = \sqrt{243} = 9 \sqrt{3} [/tex]
