To raise $$3,500,000$ to expand into new markets, a very successful laptop manufacturing company issued bonds in the market with a coupon rate of $7\mathrm{.}50\%,$ paying interest every $6$ months, and redeemable in $17$ years. They established a sinking fund to retire this debt on maturity and made equal deposits into the fund at the end of every half$-$year.

a$.$ If the fund was earning $5\mathrm{.}10\%$ compounded semi$-$annually, calculate the periodic cost of the debt.

Round to the nearest cent

b$.$ Calculate the book value of the debt after $12$ years.