Question 1089881
{{{ drawing( 300, 300, -5, 5, -5, 5,
  locate(0,3.7, 2),
  locate(0,2.7, 4),
   locate(1,3.3, 3),
   line( -3, 4, 3, 2 ),
   line( -3, 2, 3, 4)

)
}}}

<pre>

Angles 2 and 3 are supplementary, therefore their measures add to 180°:   
m&#8736;2 + m&#8736;3 = 180°      
m&#8736;2 = 180°  -  m&#8736;3    (1)

Angles 4 and 3 are also supplementary, therefore their measures add to 180°:
m&#8736;4 + m&#8736;3 = 180° 
m&#8736;4 = 180° -  m&#8736;3     (2)

Compare (1) to (2).  Do you see it?   

If  a = c  and b = c  then a = b.   

I left the measure of angle 2 out of the proof because it doesn't matter what its measure is,  angle 2 and angle 4 will ALWAYS be equal (vertical angles are always equal).