Here is the question:

A spring with a 7-kg mass and a damping constant 5 can be held stretched 1 meters beyond its natural length by a force of 4 newtons. Suppose the spring is stretched 2 meters below spring-mass equilibrium and then released with zero velocity.

Find the position of the mass after t seconds.

Here are the data:
m = 7 kg
c = 5
k = 4/1 = 4
c^2 - 4mk = -87 (Under-damped)

Using the formula:
w = sqrt(4mk - c^2)/2m
= sqrt(87)/14

x = e^-(c/2m)t(C1Cos(wt) + C2Sin(wt))

and I think the initial values are:

x(0) = 2 x'(0) = 0

so I got

x(t) = e^-(5/14)t(2Cos(sqrt(87)/14*t))

But it's wrong!!

Please help!!