Need to find the greatest common factor

What is the point-slope equation of a line with slope -5 that contains the point (6,3)

For the rule of 78, for a 12-month period, the last term in the sequence is 12 and the series sums to 78. for an 10 month period, the last term is and the series sum is . for a 15 month period, the last term is and the series sum is . for a 20 month period, the last term is and the series sum is

How do you solving the solution of each system using elimination?