I’m struggling a bit on this question. I could use some help please. Number 12

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

Ined this is due tomorrow and i dont know the answer can you find all the exponents

What values of x satisfy the inequality x^2-9x< 0? how is your answer shown on the graph?