posted by bob .
if earth is one astronomical unit from the sun and has a period of 1 year, approximately how far is planet X form the sun if it has a period of 26 yearS
Kepler's third law says that R^3/P^2 = constant.
We can choose to measure P in years and the orbital radius R in a.u.
In your case, the ratio (R^3/26^2) = 1^3/1^2 = 1
R^3 = 676
R = 8.8 a.u.