Halley’s comet has a perihelion distance of 0.6 au and an orbital period of 76 years. what is its aphelion distance from the sun?

Aphelios derived from the Greek word, “apo” means far and “helios” meaning sun, it means far away from the sun.
To solve:
The Period  gives the semi-major axis of the orbit:

a3 = P2 a = P2/3 = (76 AU)2/3 = 17.9 AU
so…
ra + rp = 2a ra = 2a - rp = 2(17.9) - 0.6 = 35.2 AU

Therefore, Halley's Comet stretches maximum distance from the Sun between the orbits of the planets, Neptune and Pluto.