Since this is the proof I will only do explanation:
Step-by-step explanation:
If m∠1=a and m∠3=b:
a+50°=180° because they are supplementary
and b+50°=180° because they are supplementary
Because of this:
a+50°=b+50°=180°
Subtract 50°:
⇒ a=b=130°
They are equal and they equal 130°
