-3.5 means you start the game 3 and a HALF points down or you are giving 3 and a half points.... plus 3.5 you are getting 3 and a half its the spread. You must determine if a team can cover the spread or do you want the points and the underdog.
example i think you took Miami - 3.5 you are giving Houston 3.5 to start Miami must win by 4 to cover the spread. If the final is Miami 21 Houston 18 Miami loses because they didn't cover, Houston would win by a half keeps it where there is no ties, Miami wins the game u lose the bet.