The after-tax rate of return on a taxable bond is calculated by multiplying the pre-tax yield by one minus the tax rate. In this case, it would be 9.1% x (1 - 0.21) = 7.19%. The municipal bond is tax-free, so its after-tax rate of return is equal to its pre-tax yield of 7.35%. Therefore, you should buy the municipal bond as it has a higher after-tax rate of return than the taxable bond.