a. Given (This is part of the original problem) b. Supplementary angles form 180 degrees (The definition of supplementary angles is that two or more angles have a sum of 180 degrees. DC is a straight line and has a measure of 180 degrees) c. Substitution (Given the two previous statements, they are each equal to 180 degrees. Since 180=180 then you can substitute the angles in the equation.) d. Subtraction (m<2 is being subtracted) e. Subtraction postulate (Since the same quantity was being subtracted from each side of the equation, they are still equal)