Alternate interior angles:
In the given figure, ∠5 and ∠1 are alternate interior angles.
Similarly, ∠6 and ∠3 are also alternate interior angles.
[tex]\angle5\cong\angle1\: and\: \angle6\cong\angle3[/tex]
Straight angles:
In the given figure, m∠1, m∠2, and m∠3 make a straight line angle equal to 180°.
[tex]m\angle2+m\angle1+m\angle3=180\degree[/tex]
Substitution:
As you can see, we have already established that ∠5 = ∠1 and ∠6 = ∠3
Let us substitute this into the above equation so it becomes
[tex]m\angle2+m\angle5+m\angle6=180\degree[/tex]
