Please help……I do not understand this

Use the elimination method to solve the systems of equations. choose the correct ordered pair. 7x+4y=39

An office supply company conducted a survey before marketing a new paper shredder designed for home use. in the survey, 80 % of the people who tried the shredder were satisfied with it. because of this high satisfaction rate, the company decided to market the new shredder. assume that 80 % of all people are satisfied with this shredder. during a certain month, 100 customers bought this shredder. find the probability that of these 100 customers, the number who are satisfied is 69 or fewer.

What is the missing reason in the proof? segment addition congruent segments theorem transitive property of equality subtraction property of equality?