(5,6) and (6,8) equation of line

What is the inverse of the function f(x)=2x+1

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 negative 3/9 times negative 2/8 ?