Evaluate 2/3x + 1/2 for x=3/4 (I just copied the exact question)

Which is an equation for the nth terms of the sequence 12,15,18,21

Find the value of x. also give me an explanation so i can complete it to other questions as well, : )

You are choosing between two health clubs. club a offers membership for a fee of $ 28 plus a monthly fee of $ 20. club b offers membership for a fee of $ 20 plus a monthly fee of $ 22. after how many months will the total cost of each health club be the same? what will be the total cost for each club? in __ months the total cost of each health club will be the same.