Use the sine:
[tex]sine=\dfrac{opposite}{hypotenuse}[/tex]
[tex]\sin18^o=\dfrac{x}{85}\\\\\sin18^o\approx0.309\\\\\dfrac{x}{85}=0.309\ \ \ \ |\cdot85\\\\\boxed{x=26.265\approx26.3}[/tex]
