Answer
$$ - 1.5398$$
Work Step by Step
$$\eqalign{
& \tan {123^ \circ } \cr
& {\text{using a calculator to find the value }}\left( {{\text{set the calculator in degree mode}}} \right) \cr
& {\text{then}}{\text{, we obain}} \cr
& \tan {123^ \circ } \approx - 1.5398 \cr} $$