$******3\mathrm{.}2\mathrm{.}6.$ Do the following sets of equations have a unique solution?

a$.$

$u+v+w=0$

$u+2v+3w=0$

$3u+5v+7w=1$

b$.$

$u+w=0$

$5u+4v+9w=0$

$2u+4v+6w=0$