You would begin by choosing which trigonometric function to use.
In this case, you would use sine.
Sine is opposite over hypotenuse.
Opposite of the 35 degree angle is 18, and you want to find the hypotenuse which is x.
sin(35) =
(multiply x to both sides)
x sin(35) = 18 (divide sin(35) to both sides)
x = 31.38204...
So, therefore
x = 31.38. I don't know what decimal point you're supposed to round to, so I'm gonna guess the hundredths place. :)