How to find local extrema with second derivative test?

With these: 18/36 = 1/? missing number change 1 5/8 to improper fraction. change 19/5 to a mixed number.

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

Find the value of x for which the lines are parallel.