Live 10/20 5 handed

[LA_Price]

an ok 10/20 game has just turned into a great shorthanded game as nobody in the game has any clue how to play at a less than full table. Tons of open limping etc...

anyways I open 55 in the CO and get threebet by the button who I would describe as abc weak passive. Whats my plan for the hand. Is it flop a set or fold? or is it board dependent.

Flop came T84. is this a clear check and fold situation? or should i call down. Would your answer be different if i had 99? I