Answer
$t=0; t=\dfrac{\pi}{2}; t =\pi$
Work Step by Step
$v(t)=\dfrac{dr}{dt}=3 \cos t \ k -5 \sin t j$
and $a(t)=\dfrac{dv(t)}{dt}=-3 \sin t \ k -5j \cos t $
Now, $v(t) \times a(t)=(3 \cos t \ k -5 \sin t j) \times (-3 \sin t \ k -5j \cos t) =16 \sin t \ \cos t $
When $v \times a=0$
This implies that $16 \sin t \ \cos t =0$
or, $\sin t =0$ or, $\cos t=0$
This implies that $t=0; t=\dfrac{\pi}{2}; t =\pi$
