Suppose high-grade taxable bonds trade at a 12% yield to maturity and high-grade tax-exempt municipal bonds with the same maturity trade at a 8% yield to maturity. If equityholders are taxed at a 15% rate and corporations are taxed at a 34% rate, what is the PV(Tax Shields) per dollar of debt?

Pleas help me out here, I’m studying for my finance exam but I don’t know how to do this problem.