Given:
[tex]\angle1\text{ and }\angle2\text{ are vertical angle}[/tex]
Sol:.
[tex]\begin{gathered} m\angle1+m\angle3=180 \\ m\angle2+m\angle3=180 \end{gathered}[/tex]
use both equation then use substitution property then.
[tex]m\angle1+m\angle3=m\angle2+m\angle3[/tex]
