If x = a ≠ 0 is a singular point of a second-order linear differential equation, then the substitution t = x - a transforms it into a differential equation having t = 0 as a singular point. We then attribute to the original equation at x = a the behavior of the new equation at t = 0. Classify (as regular or irregular) the singular points of the differential equations in Problems 9 through 16.

(x² - 9)² y" + (x² + 9)y' + (x² + 4)y = 0