Mercury is closest to the sun but venus is the hottest planet.why? answerr fast...plss

This is due to the atmosphere difference that both the planet experience. Venus is able to trap the heat but mercury is not able to do that. It depends on how much heat it generates by itself, how much heat it receives from outside, and how much heat it retains. Mercury is much closer to the sun, so it receives more heat. 
But mercury has no atmosphere,so it has nothing to trap the heat all the heat is radiated back to univese.
Venus has a very thick & acidic atmosphere that is mostly carbon dioxide. heat is dissipated through infrared light, and CO2 is opaque to infrared light. so the infrared, which is heat, is trapped.