Sine law is used when yow have a triangle which you know an angle and it’s opposite side as well as another side or angle. At the top of the next immage you will see the 3 equations that you use to solve the triangle.
As you can see we are looking for side a in this triangle. Since we have not B angle or b side we will not have to use the sine law b equation. The next picture shows the equations filled in.
From here you just have to solve the equation.