What Is Law of Sines?
The law of sines states that in any triangle, the ratio of a side's length to the sine of its opposite angle is the same for all three sides: a/sin(A) = b/sin(B) = c/sin(C). It's used to solve triangles that aren't right triangles, when you know either two angles and a side, or two sides and a non-included angle.
Example
In a triangle with angle A = 30°, angle B = 105°, and side a = 10, side b can be found with b = a × sin(B)/sin(A) = 10 × sin(105°)/sin(30°) ≈ 10 × 0.9659/0.5 ≈ 19.32.