$.$ Evaluate the following limits or prove that the limit does not exist by giving two paths toward the limit point with different limits$.($a$)[5$ points$]$ lim $($x$,$y$)(2,1)$ x $+2$y $42$x $4$y $($b$)[5$ points$]$ lim $($x$,$y$)(0,0)4$xy$33$x $2+3$y $2$