|Mark 9:33a And he came to Capernaum:||Matt 17:2424a And when they were come to Capernaum,|
Matt 17:24b they that received tribute money came to Peter, and said, Doth not your master pay tribute ?
Matt 17:25a He saith, Yes.
Matt 17:25b And when he was come into the house, Jesus prevented him, saying, What thinkest thou, Simon? of whom do the kings of the earth take custom or tribute? of their own children, or of strangers?
Matt 17:26a Peter saith unto him, Of strangers.
Matt 17:26b Jesus saith unto him, Then are the children free. 27 Notwithstanding, lest we should offend them, go thou to the sea, and cast an hook, and take up the fish that first cometh up; and when thou hast opened his mouth, thou shalt find a piece of money: that take, and give unto them for me and thee. 
An explanation of "tribute money" can be found in Exodus 30:13 This they shall give, every one that passeth among them that are numbered, half a shekel after the shekel of the sanctuary: (a shekel is twenty gerahs:) an half shekel shall be the offering of the Lord (New Oxford Annotated Bible, p. 1194).
Compare this to Mark 12:13-17.