# If you know that the period is 76 years and the semi major axis is 17.94 AU, how do you calculate the distances of the perihelion and aphelion?

The period $P$ and semi-major axis $a$ are not sufficient to calculate the apses distances as the eccentricity of the orbit can't be determined.
If the eccentricity $e$ is known then the perihelion distance is $a \left(1 - e\right)$ and the aphelion distance is $a \left(1 + e\right)$.
In fact the period and semi-major axis can be calculated from each other using Kepler's third law. For our solar system if $P$ is in years and $a$ is in AU then ${P}^{2} = {a}^{3}$.