Solution 3.8.1.13 Applying the Laplace transform, assuming zero initial conditions, yields s 4 Y (s)+15s 3 Y (s)+74s 2 Y (s)+135sY (s)+72Y (s)=U(s)+sU(s);; or (s 4 +15s 3 +74s 2 +135s +72)Y (s)=U(s)(s +1): Rearranging yields Y (s) U(s) = s +1 s 4 +15s 3 +74s 2 +135s +72 : 1