Where’s the plain scripture declaring a 10,000 year old Earth?
There isn't any, because it assert a ~6000 year old earth!
Premises:
1. A day is 24 hours in earth time
a year is one orbit of the earth about the sun,
3.The genealogies from Adam to Abraham are correct, as confirmed in Matthew 1 and Luke 2.
If you study Scripture, especially the Geneologies of Genesis 5 & 11, you arrive at God's covenant with Abram 2083 years after Adam's making.
Add 430 years in Egypt (Ex 12:41, Gal 3:16-17), 480 years to Solomon's 4th year of his reign(1 Kings 6:1), and you get to 2083+910 = 2993 After Adam's making.
Non-biblical archeology places Solomon's birth ~1000 BC, thus 2993+1000 = ~3993 years after creation = 1 AD.
Add 2015 years and the Earth is around
6000 years old (6008?)
Many assert that there are missing generations in Genesis, and thus they arrive at 10,000 years old. But like I mentioned, I take it literally.