What is the answer to 2(0.5c−1.5)−6=−2 show

What is the approximate circumference of the circle shown below?

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

Which expression is equivalent to the expression shown?