Suppose the Vector Field $\vec { F } ( x , y )$

Essay

Suppose the vector field $\vec { F } ( x , y )$ represents an ocean current and that $\vec { r } ( t ) = 8 \cos t \vec { i } + 9 t \sin t \vec { j }$ is a flow line of $\vec{F}$ Find the acceleration vector of the flow line at (0, 9 $\pi$ /2).