The factor of mn+5m-3n-15 is 6m-4n-15.
-4/5 is the skip of the graph
Reason F should be "CPCTC" which stands for "corresponding parts of congruent triangles are congruent". Its like saying "if two houses are identical, then the front doors should be the same". The houses in the analogy are the triangles, while the front doors are the corresponding parts. So if triangle DEC is congruent to triangle BEC, then the corresponding parts angle DEC and angle BEC are congruent.
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Reason H is "linear pair postulate" which says that if two angles form a linear pair then they are considered supplementary. This is simply what "supplementary" means. The two angles add to 180 degrees. A "linear pair" is where you have two angles that are adjacent and the angles combine to form a straight angle (180 degrees).
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Side note: It seems like some of this proof has been cut off. There should be more lines to this proof because the last line is always what you want to prove. In this case, the thing we want to prove is "angle DEC and angle BEC are right angles" so that should be the last statement.
Parallel never cross
perpendicular is a 90 degree angle
its neither if its not 90 degree or never crossing
Here's our expression: 
We will follow the order of operations and begin by solving everything in parentheses. 
Next, we solve the exponents. 
Now, we solve the multiplication and division. There is no division, so we will just solve the multiplication. 
Finally, we solve the addition and subtraction. Since there is no addition, we will solve the subtraction. 
