Find y as a function of t if 9y'' - 31y = 0, y(0) = 4, y'(0) = 6.

a) y(t) = 2cos(2t) + 2sin(2t) + 4
b) y(t) = 2sin(2t) + 2cos(2t) + 4
c) y(t) = 3cos(2t) + 3sin(2t) + 4
d) y(t) = 3sin(2t) + 3cos(2t) + 4