We have the following:
when two angles are supplementary, the sum of these is equal to 180 degrees, therefore
[tex]<3+<4=180[/tex]
In this case, <3 is 56°, replacing:
[tex]\begin{gathered} 56+<4=180 \\ <4=180-56 \\ <4=124 \end{gathered}[/tex]
Therefore, the angle <4 measures 124 °
