Since we have a right triangle, we can apply Pythagorean theorem. The hypotenuse is always in from of the righ angle, then we can write
[tex]18^2=x^2+9^2[/tex]which gives
[tex]324=x^2+81[/tex]By subtracting 81 to both sides, we have
[tex]\begin{gathered} 243=x^2 \\ or\text{ equivalently,} \\ x^2=243 \end{gathered}[/tex]By taking square root to both sides, we get
[tex]\begin{gathered} x=\sqrt[]{243} \\ x=15.588 \end{gathered}[/tex]Therefore, by rounding to the nearest tenth, the answer is 15.6 units