What is the average rate of change of the function f(x) = -3x2 + 7x + 15 over the interval -2 sxs 2?

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