Final answer:
The question seeks to solve an initial value problem involving a differential equation, but the provided substitution information is unclear, making the intended solution method difficult to determine without further context or clarification.
Step-by-step explanation:
The question at hand requires solving the initial value problem yy' + x = √x²+y with the given initial condition y(3) = -√40. The solution to this kind of differential equation typically involves a substitution method, where a new variable M is introduced to simplify the equation.
However, the provided information does not clearly indicate a specific substitution to be used, and it seems there might be a misunderstanding or typo in the question. Given the typical methods for solving such equations, potential substitutions could be M = y² or M = yy', but without the proper context or equation form, it is difficult to determine the intended approach.
Since the provided references like dx²-y² and other referenced equations do not directly relate to the initial problem, and as no substitution seems immediately applicable, it would be unprofessional to guess a solution. We would need more information or clarification on the intended method of substitution before proceeding with the solution.