Let $n$ be the length of the stick which is broken off. The length of the stick which is not broken off will therefore be $15-n$. Note that the stick which is broken off is the hypotenuse of a right triangle, while the stick which is not broken off is one of the legs. Also, notice that the sine function is the **opposite over the hypotenuse**. So if $n$ is the hypotenuse and $15-n$ is the side opposite the angle, we get that
$$\frac{15-n}{n} = \sin{60^o}$$
which, using the unit circle evaluates to $\sqrt{3/4}$. Now we get the equation
$$\frac{15-n}{n} = \sqrt{3/4}$$
which simplifies to
$$15-n = n\sqrt{3/4}$$ $$15 = n + n\sqrt{3/4}$$ $$15 = n(1 + \sqrt{3/4})$$ $$n=15(1+\sqrt{3/4})$$
Therefore, the hypotenuse is $15(1+\sqrt{3/4})$. Solve from here.