Since it is a right triangle, we can use the Pythagorean theorem, as follows:
[tex]a^2+17^2=18^2[/tex]
And solve for a:
[tex]\begin{gathered} a^2+289=324 \\ a^2+289-289=324-289 \\ a^2=35 \\ a=\sqrt{35}=5.9 \end{gathered}[/tex]
Answer: missing side = 5.9
