Here is the problem:

A frictionless spring with a 8-kg mass can be held stretched 1.2 meters beyond its natural length by a force of 30 newtons. If the spring begins at its spring-mass equilibrium position, but a push gives it an initial velocity of 1.5 m/sec, find the position of the mass after t seconds.

So I organized the data and find that

m = 8 kg

K = 30N/1.2M = 25 N/m

and that

External Force = 1.5 m/s

How do I approach this problem?

I think I should use the formula

mx'' + kx = F0cos(wt) but not sure how to

Thanks for the help!!