Answer
$60^{\circ}$
Work Step by Step
Supplement of $120^{\circ}$ will be another angle that terminal side needs to sweep in order to reach at standard position of $180^{\circ}$. (Figure 24)
Obviously it will be $60^{\circ}$.
Another way to find supplement of an angle will be to subtract it from $180^{\circ}$.
Hence supplement of $120^{\circ}$ = $180^{\circ}$- $120^{\circ}$ = $60^{\circ}$
