A current distribution gives rise to a vector magnetic potential A $=$ x$2$yax $+$ xy$2$ay $$

$4$xyzaz Wb$/$m$.$ Obtain following

$($a$)$ Magnetic flux density B at $(-1,2,5)(15$ points$)$

$($b$)$ The flux through the surface z $=1,0$ x $1,-1$ y $4(15$ points$)$