The 39° angle is opposite the given 8, and the side you’re looking for is the
hypotenuse, so you can use the sine to find x.
Now, nobody expects you to know the sine of 39° off the top of your head. This
is one of those places where you have to use your calculator. Punch in “8 ÷ sin
39 =,” making sure your calculator is set to degree mode, and you’ll get
something like: 12.712125825, which is close to (D) 12.7.