Hi Mackenzie.
The municipal bonds are paying 4%.
To determine the equivalent taxable rate, we want a rate which after paying federal taxes (22%) and state taxes (3%) will yield 4%.
.22+.03 = .25
1-.25 = .75
4% / .75 = 16%/3 = 5.33%
With a 22% federal tax rate and a 3% state tax rate, a 5.33% taxable return is equivalent to a 4% municipal bond return.