For Problems, verify that the given functions y1 and y2 are solutions of the given differential equation, then show that c1 y1 (t) + c2 y2 (t) is also a solution for any real number c1 and c2.
a. y′′ − 9y = 0; y1 = e3t y2 = e-3t
b. y′′ + 4y = 0; y1 = sin 2t y2 = cos2t