Which Trig. Function do you use to find X?

Which of the following are parts of a sphere

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 solution to the equation below? 3/x-2+6=square root x-2 +8