P3.32 Extender/robot state variable

I have a Transfer function that is:

Y(s)/U(s) = G(s) = 30/(s^2 + 4s + 3)

U(s) is the force of a human hand applied to a robot manipulator
Y(s) is the force of the robot manipulator applied to a load

I used a long-hand method to get y(t).

I came up with an answer that y(t) = 15e^(-t) - 15e^(-3t) by using partial fraction solutions and Inverse Laplace.

However the plot does not look correct.  

I need help to determine what y(t) should be and how to use the long-hand method to get the correct answer.

Thanks,