I need help with this problem

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

What is the ratio of the length of an object in yards to the length of the same object in miles?

Sole the equation 6n + 9 = -3n - 21 - 6 for n