When you indicate triangle congruency, the order of the letters matter. As you see these statements, look at how the side length names correspond to how the letters are written in its order, ΔABC ≅ ΔDEF. (By the way, triangles aren't equal with an = sign. It's a ≅ congruency sign.)
AB is congruent or the same length as DE.
BC is congruent or the same length as EF.
AC is congruent or the same length as DF.
In the same way, all of the angles in the corresponding order are ALSO congruent. By the way, we say that angles are congruent and angle MEASURES (like 30 degrees) are equal.
m∠A = m∠D
m∠B = m∠E
m∠C = m∠F
In the same way, we can try to find m∠C, or measure C!
1. Let's find the missing angle!
So, we know that m∠A is 50 degrees and m∠E is 30 degrees. Since we know that m∠E = m∠B because of the order of the triangles, now we know two of the angles in triangle ABC.
m∠A = 50 degrees
m∠B = 30 degrees
m∠C = 180 degrees - (50 + 30) = 100 degrees
(We minus from 180 degrees because 180 degrees is the sum of the angles in a triangle!)
And that's it! If you have any questions, please feel free to ask questions. I'm not here to judge, and I'm only here to help. Again, ask questions if you need help. It's crucial that you know the basics of geometry!