Based on the given figure, prove that if a transversal intersects two parallel lines, then alternate interior angles have equal measure.Question: What is the missing statement in Step 3?

The angle c and angle g are pair of corresponding angles. So,
[tex]\angle c\cong\angle g[/tex]As angle c and angle g are equal. So sum of angle f and angle g can be expressed as sum of angle f and angle c.
Thus missing statement for step 3 is,
[tex]\angle c=\angle g[/tex]