Your answer x = 5 sin (30) / sin(90) is correct, you just need to take the last step of plugging in what sin(30) and sin(90) are. Using law of sines is not the usual way to solve this problem but its not incorrect. (Law of Sines applies to ANY TRIANGLE and is typically used when the triangle is NOT a right triangle and its the only method that will work. Since this IS a right triangle you can just consider sine or cosine directly as the other commenter demonstrated)
You should know the values for sine, cosine, tangent of the basic angles like 0, 30, 45, 60, 90 degrees. Those are ones that teachers traditionally expect you to memorize.