Answer:
77
Explanation:
Let's let the unknown number be x.
Supplementary angles add up to a total of 180 degrees. Since we already know one angle is 103, this means that:
![103+x=180](https://img.qammunity.org/2021/formulas/mathematics/college/2g4edff8lvwrkak42oe9gbwdgfh24qfgdg.png)
Subtract 103 from both sides:
![x=77\textdegree](https://img.qammunity.org/2021/formulas/mathematics/college/3kmc138k2i6wvyjbvhjzhok9wrfzyxdu5g.png)
So, the measure of its supplementary angle is 77 degrees.
And we are done!