This is a right triangle. There is no need to use the law of sines.

You are given the hypotenuse length and the angle between S1 and the hypotenuse.

From this and the definition of the cosine function

cos(angle between S1 and hypotenuse) = S1/hypotenuse so

cos(40deg) = S1/15

you can solve this for S1. You can then use either sin(40deg) or the Pythagorean theorem to solve for S2.