Slower
Step-by-step explanation:
Kepler's harmonic law (also called Kepler's third law) states that:
"The cube of the radius of the orbit of a planet around the sun is proportional to the square of its period of revolution"
In formula, this can be written as
where
r is the radius of the orbit
T is the orbital period
const. is a constant number, which is the same for all the planets orbiting the sun
By looking at the equation, we can see that the further a planet from the sun, the larger its orbital period: this means that the further planet will have a slower orbit. In this problem, we have to compare Neptune and Mercury: since Neptune is further from the sun than Mercury, also its orbital period will be larger, and therefore Neptune would have a slower orbit around the Sun than Mercury.