$\displaystyle y=2cos(\frac {x}{2}) $, $\displaystyle \frac {d^2y}{dx^2} $
any hints were to begin
First find the first derivative via chain rule:
$\displaystyle \begin{array}{rcl}\displaystyle \frac{dy}{dx} & = & \displaystyle -2\sin \left(\frac{x}{2}\right) \cdot \left(\frac{x}{2}\right)' \\ & = & \displaystyle -2\sin \left(\frac{x}{2}\right) \cdot \frac{1}{2} \\ & = & \displaystyle -\sin \left(\frac{x}{2}\right) \end{array}$
Now apply the chain rule again to find the second derivative.