( Very Embarrased, and sad! I just had my very 1st period to day, so I had to tell my Male Teacher, and he?
Use Kepler's 3rd Law: T^2 directly proportional to R^3 Since (5/365.25)^2 = 0.0001874 the semi-major axis is (0.0001874)^(1/3) AU 0.0572 AU (about 5.3 million miles).
A (in AU) = 5/365.25²/³ = 1/73.05²/³ = 1/17.47 = 0.057225 AU = 8,560,874 km.