When we use the method of Undetermined Coefficients to find a participation solution for a given differential equation (non-homogeneous), it is limited to the case that f(a) is in the form of sine function, cosine function, exponential function, polynomial and product/sum of the functions mentioned. This method won't work if f(x) = sec 3x, for example, y" + 9y = 2 sec 3x. Then we will use another method called Variation of Parameter. Let's use this method to solve the differential equation y" + 9y = 2 sec 3x