at http://tutorial.math.lamar.edu/Classes/DE/SecondOrderConcepts.aspx in first example the author says that "We need functions whose second derivative is 9 times the original function. " and then after giving two satisfactory solutions, he gives bunch of other solutions with this statement "These two functions are not the only solutions to the differential equation however. Any of the following are also solutions to the differential equation." but none of these bunch of solution is a function whose second derivative is 9 times the original function. Then how can we say that they are the solu?

Question

at http://tutorial.math.lamar.edu/Classes/DE/SecondOrderConcepts.aspx in first example the author says that "We need functions whose second derivative is 9 times the original function. " and then after giving two satisfactory solutions, he gives bunch of other solutions with this statement "These two functions are not the only solutions to the differential equation however.  Any of the following are also solutions to the differential equation." but none of these bunch of solution is a function whose second derivative is 9 times the original function. Then how can we say that they are the solu?