1) y''(t) + 5 y'(t) + 6 y(t) = exp(-t)u(t), y(0) = 1, y'(0) = -1. Solution: y(t) = [ .5 e^{-t} + e^{-2t} - .5 e^{-3t} ] u(t) 2) y''(t) + 9 y'(t) + 20 y(t) = cos(t)u(t), y(0) = 0, y'(0) = 0.