If a computer company switches to a more efficient hard disk drive, the company will save 3,500 dollars every 6 month in repair cost

If a computer company switches to a more efficient hard disk drive, the company will save 3,500 dollars every 6 month in repair cost. What single cash investment made now is equivalent to the repair payments if the interest is 8% compounded quarterly and the payments are being made

a. After the closing of the every 6th month for 15 years?

b. at the start of every 6 months for 9 years - but has been deferred for 3 years?

c. at the closing of every 6 months in perpetuity?

d. at the start of every 6 months in perpetuity?

Please use TI BA II calculator features (N, I/Y, PV, PMT, FV, AMORT) to solve questions (if possible)