I Don't Know How To Solve This Physics Problem

Question

Duende has an elliptical orbit about the Sun, with a perihelion distance of 0.829 AU and aphelion distance of 0.992 AU. What is the period of its orbit (in years)?

Answer 1

Average radius orbit R = (0.829 + 0.992)/2 = 0.9105 AU.

For Earth radius orbit R₀ = 1 AU and period T₀ = 1 year.

By 3rd Keppler's Law: T²/ T₀² = R³/R₀³. T = T₀(R/R₀)^3/2 = 1year·(0.9105AU/1AU)^3/2 = 0.8688 years