First, draw out this triangle and label the appropriate sides and angles. The angle side with a length of 24 is adjacent to the 35° angle, so it should be between this angle and the right angle of the triangle.
Since your goal is to find the length of the side across from the 35° angle, and you know the adjacent sign, the best formula to use is tangent, or opposite over adjacent. Now you just have to plug in the information you already have until you have tan(35°) = x / 24
x / 24 is the side you are trying to find / 24, or opposite / adjacent.
To solve for x, multiply both sides by 24, so now you have x = 24*tan(35°) = 16.8
If you have more questions let me know!