March 21st is an equinox. what part of earth is directly beneath the sun on that day?

Equator from planet Earth.

Explanation:

During the equinox of March 21 the sun is on the planet's equator for this reason at this time the day and night have the same duration

By this date the Sun reaches its zenith, or the highest point in the sky, at 90 ° in relation to a person on Earth, so the parallel of the decline of the Sun and the celestial Ecuador coincide that day.

this age may represent the age of the earth's accretion, of core formation, or of the material from which the earth formed. this dating is based on evidence from radiometric age-dating of meteorite material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.