How do you solve a two column geometric proof about the triangle exterior angle theorem?

Question

Prove the exterior angle is equal to the sum a the remote interier angles 

Answer 1

Label the measurements of any triangle's three angles as a, b and c.

Extend one side to show that the exterior angle associated with interior angle c measures 180-c.

 

a+b+c = 180, for sure!

Subtract c from each side: b+c = 180-c. 

This proves that the measure of the exterior angle equals the sum of the measures of the two REMOTE INTERIOR ANGLES.

 

Surely you can arrange this into 2-column form.