Final answer:
To solve the non-linear differential equation dy/dx = (x² + y² + 9), an advanced substitution method not covered in this discussion is required.
Step-by-step explanation:
To solve the given differential equation dy/dx = (x² + y² + 9), an appropriate substitution must be used. Generally, there are standard methods for solving differential equations of the form dy/dx = f(ax + by + c). However, this specific equation does not appear to be linear, separable, or fit into another common form directly. Hence, it may require a more advanced method or substitution. If one is not familiar with the method or if the equation is not covered by the course, it is advisable to consult a mathematics textbook or a resource that specializes in differential equations to find the appropriate substitution or transformation that can simplify this equation into a solvable form.