Step 1: Definition of the degree of a differential equation.
The degree of a differential equation is determined only when the equation is a polynomial in all of its derivatives.
Step 2: Examine the given equation.
The provided equation is:
\[
(y'')^2 + (y')^3 = x \sin(y').
\]
This equation includes a non-polynomial term, \( \sin(y') \), where \( y' \) is a derivative. Therefore, the degree of the equation cannot be defined.
Step 3: Final Answer.
Thus, the degree of this differential equation is:
\[
\boxed{\text{Not defined}}.
\]