a
$\left| {\begin{array}{*{20}{r}} 1&1&1\\ 3&2&2\\ 9&4&{28 + [\lambda ]} \end{array}} \right|$ $=-24-[\lambda] +15=-[\lambda]-9$
if $[\lambda]+9 \neq 0$ then unique solution
if $[\lambda]+9=0$ then $\mathrm{D}_{1}=\mathrm{D}_{2}=\mathrm{D}_{3}=0$ so infinite solutions
Hence $\lambda$ can be any real number.