$\omega=2\pi f=2\pi . \dfrac{5}{\pi}=10(rad/s)$
$A=\dfrac{a}{\omega^2}=\dfrac{6}{10^2}=0,06m=6cm$
$a) t=0; x=0 (+) \Rightarrow \varphi=-\dfrac{\pi}{2}$
$x=Acos(\omega t+\varphi)$
$=6cos(10t-\dfrac{\pi}{2})$
$b) v=-\omega Asin(\omega t+\varphi)$
$=-60sin(10t-\dfrac{\pi}{2})$
$c) a=-\omega^2 x=-600cos(10t-\dfrac{\pi}{2})$