differential equation for newton's second law

So i have the equation m*(d2x/dt^2)+c*(dx/dt)+kx=0, where d2x/dt^2 is the second derivative.

So i'm given that m=10 kg, and k=28 N/m. At time t=0 the mass is displaced to x=.18m and then released from rest. I need to derive an expression for the displacement x and the velocity v of the mass as a function of time where

a) c=3 N-s/M

b) c=50 N-s/m

Since I have to do this in matlab, I attempted to solve with dsolve and got

C10/exp((t*(c - (c^2 - 4*k*m)^(1/2)))/(2*m)) - (C10 - 9/50)/exp((t*(c + (c^2 - 4*k*m)^(1/2)))/(2*m))

clearly not right..How do i set it up correctly so i can solve for both x and v?

Thanks