Question 1064516
I assume you have a picture that looks more or less like this:
{{{drawing(350,300,-1.5,5.5,-0.5,5.5,
triangle(0,0,5,0,4,5),triangle(0,0,4,5,-1,5),
locate(-0.1,0,A),locate(4.95,0,B),
locate(3.9,5.3,C),locate(-1.1,5.3,D)
)}}} Because AB, and CD are parallel,
and intersected by transversal AC,
alternate interior angles CAB and ACD are congruent.
Since AB is congruent to CD,
and AC is congruent to itself,
triangles ABC and CDA are congruent
by SAS (side-angle-side).
 
If your figure looked like this,
{{{drawing(500,300,-0.5,9.5,-0.5,5.5,
triangle(0,0,5,0,4,5),triangle(0,0,4,5,9,5),
locate(0,0,A),locate(5,0,B),
locate(4,5.3,C),locate(9,5.3,D)
)}}} with AB parallel to CD,
and congruent with CD,
ABC would not be congruent with CDA,
but it would be congruent with DCB instead.