How is jesus both the seed of david and the son of god?

The Bible itself says that Jesus is the "son of God", the"first born of all creation"(Matthew 16:16)(Colossians 1:15; Revelation 3:14). He was in heaven long before being sent to earth by his father, to be born as a perfect human child(Luke 1:35)(John 3:13)(John 6:38)(John 8:23)

He was sent for a specific purpose: To 'ransom' mankind (Matthew 20:28)and pay the price for Adam's sin(Romans 5:12, 18,19, 23); Perfect human life for perfect human life.

The Jewish nation was set up to supply the 'Messiah' in the flesh.

This Jewish nation was made up of twelve family lines which came from Abraham, Isaac, and Jacob, and the line chosen for the Messiah(Shiloh) was Judah, the line of kings.(Genesis 49:10)

David was from this line, and was a sincere and faithful worshiper of YHWH, the God of the Israelites(John 8:54) and prefigured the Messiah(Isaiah 11:1-5, 10; Romans 15:12) because of his sincere faith and strong adherence to God's laws. Even though he made some serious mistakes, his strong faith was rewarded by his being included in Messiah's lineage(Matthew 1:1+6). Joseph and Mary were BOTH from David's family, through different sons, so in a fleshly sense Jesus was born also in the 'line of David' (as a descendant (seed) .

These facts give Jesus the spiritual, legal and natural right to be King of God's Kingdom. (Luke 1:32, 33)