$\noindent Note the angular frequency parameter satisfies $n^2 = 256\Rightarrow n = 16$. Now, clearly the amplitude is $A = 2$ (because we pull the mass down by 2 cm and then just release), so using $v^2 = n^2\left(A^2 -x^2\right)$, the speed at the centre of motion satisfies $v^2 = n^2 A^2$...