Solution (source code)

= Solution

<Implicit differentiation> gives $2y'=15x^2+(\cos y)y'$. Therefore
$$
\boxed{y'=\frac{15x^2}{2-\cos y}.}
$$
For real $y$, the denominator is at least one, so division introduces no singularity. In fact the function $2y-\sin y$ is strictly increasing, consistent with a unique real branch $y(x)$.