Problem $7$: Consider the two$-$dimensional incompressible velocity potential $=+22.$

$($a$)$ Is it true that $2=0,$ and, if so$,$ what does this mean? $($b$)$ If it exists, find the stream function

$($x$,$ y$)$ of this flow. $($c$)$ Find the equation of the streamline that passes through $($x$,$y$)=(2,1).$