We know that $x=a \sin \omega t$ $...(1)$
Where $x$ is displacement and a is amplitude.
Velocity $=\frac{d x}{d t}=a \omega \cos \omega t$
$=a \omega \cos (-\omega t)=a \omega \sin \left(\frac{\pi}{2}-(-\omega t)\right)$
$=a \omega \sin \left(\omega t+\frac{\pi}{2}\right)$ $...(2)$
From equation $( 1 )$ and $(ii)$ it is clear that
Velocity is ahead of displacement $(x)$ by $\frac{\pi}{2}$ angle.

$Y = A \sin (\pi t +\phi)$, where time is measured in $second$.
The length of pendulum is .............$cm$