WHERE DID JESUS GIVE HIS FIRST SERMON?

Matthew 5:1. And seeing the multitudes, he went up into a mountain: and when he was set, his disciples came unto him: (KJV) Luke 6:17 And he came down with them, and stood in the plain, and the company of his disciples, and a great multitude of people out of all Judaea and Jerusalem, and from the sea coast of Tyre and Sidon, which came to hear him, and to be healed of their diseases; (KJV)

However, neither book specifically says that the sermon given was Jesus' first.  For example, Jesus could have given the sermon on the mount before the sermon on the plain, and Luke simply left out the sermon on the mount.  Leaving out information does not constitute a contradiction. Another explanation I've seen is that the wording in Matthew 5 :1 could imply that Jesus gave his sermon near a mountain, not on one, so this could be a single sermon, given on a plain in front of a mountain. Personally, I find the first explanation more likely, but, either way, this is not a contradiction.