Bond face value = 2000

semiannual coupon amount = face value*coupon rate/number of semiannual period in year

=2000*6.9%/2 = 69

semiannula years to maturity (n)=15*2 = 30

current price of bond = 2147

Bond price formula = Coupon amount * (1 - (1/(1+i)^n)/i + face value/(1+i)^n

2147 = (69*(1-(1/(1+i)^30))/i) + (2000/(1+i)^30)

Yield to maturity or pretax cost of debt is that rate where bond price will be equal to current market price. We will calculate i by trial and error method and interpolation formula. i would be semiannual rate, as coupon and periods in semiannula periods.

So Assume i=3%

Price of bond = (69*(1-(1/(1+3%)^30))/3%) + (2000/(1+3%)^30)

=2176.403972

Assume i = 3.2%

Price of bond = (69*(1-(1/(1+3.2%)^30))/3.2%) + (2000/(1+3.2%)^30)

=2095.516388

interpolation formula = lower rate +((uper rate - lower rate)*(Uper price - bond actual price)/(uper price - lower price))

3% +((3.2%-3%)*(2176.403972-2147)/(2176.403972-2095.516388))

=0.03072703301

Annual YTM =semiannual YTM*2

=0.03072703301*2

=0.06145406602

After-tax cost of debt= before tax costs of debt or Annual YTM*(1-tax rate)

=0.06145406602*(1-40%)

=0.03687243961

or 3.69%

So After-tax cost of debt is 3.69%

Note: excel function for before tax cost of debt = rate(number of periods, coupon amount, -Price of bond, face value)*2

=rate(30, 69, -2147, 2000)*2

=0.030715328*2

=0.06143065552

aftertax cost =0.06143065552*(1-40%)

=0.03685839331