(Another method)
The ball being pulled down 2cm from the origin implies that v = 0 at x = 2 or v(2) = 0. So we need an equation relating v and x.
Remember that a = -n^2 x , which means n= 16.
By chain rule,
a = \frac{dv}{dt} = \frac{dv}{dx} \cdot \frac{dx}{dt} = \frac{dv}{dx} \cdot v...