The sum of all the angles in a triangle is always 180°.
We can write the equation and solve for the missing angle:
[tex]31^o+77^o+x=180^o[/tex]
Solving for x:
[tex]\begin{gathered} x=180^o-31^o-77^o \\ \\ x=72^o \end{gathered}[/tex]
The measure of the unknown angle is 72 degrees.
