In verse 11, when Paul says, 'this is what you believed,' who is he talking to?
1 Corinthians 15:11 Therefore whether it were I or they, so we preach, and so ye believed.
The "ye" who "believed" in the resurrection from the dead, specifically of the resurrection of Jesus Christ.
Those who heard the preaching of Peter would have heard and believed this:
Acts 2:22 Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by him in the midst of you, as ye yourselves also know:
Acts 2:23 Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain:
Acts 2:24 Whom God hath raised up, having loosed the pains of death: because it was not possible that he should be holden of it.
Acts 2:25 For David speaketh concerning him, I foresaw the Lord always before my face, for he is on my right hand, that I should not be moved:
Acts 2:26 Therefore did my heart rejoice, and my tongue was glad; moreover also my flesh shall rest in hope:
Acts 2:27 Because thou wilt not leave my soul in hell, neither wilt thou suffer thine Holy One to see corruption.
Acts 2:28 Thou hast made known to me the ways of life; thou shalt make me full of joy with thy countenance.
Acts 2:29 Men and brethren, let me freely speak unto you of the patriarch David, that he is both dead and buried, and his sepulchre is with us unto this day.
Acts 2:30 Therefore being a prophet, and knowing that God had sworn with an oath to him, that of the fruit of his loins, according to the flesh, he would raise up Christ to sit on his throne;
Acts 2:31 He seeing this before spake of the resurrection of Christ, that his soul was not left in hell, neither his flesh did see corruption.
Acts 2:32 This Jesus hath God raised up, whereof we all are witnesses.
Those who heard and believed Paul's preaching, this:
Romans 4:25 Who was delivered for our offences, and was raised again for our justification.
1 Corinthians 15:3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
1 Corinthians 15:4 And that he was buried, and that he rose again the third day according to the scriptures: